What is Original Sin?
Original sin is both the lack of original righteousness and the inclination or disposition to sin. The imputation of Adam’s guilt can be thought of distinctly from original sin, but the two are integrally related; imputation is forensic, while original sin is habitual and inherent.
WCF 6:2. By this sin they fell from their original righteousness and communion with God (Gen. 3:6-8; Ecc. 7:29; Rom. 3:23), and so became dead in sin (Gen. 2:17; Eph. 2:1), and wholly defiled in all the faculties and parts of soul and body (Gen 6:5; Jer 17:9; Rom 3:10-19; Titus 1:15).
WLC Q. 25. Wherein consisteth the sinfulness of that estate whereinto man fell?
A. The sinfulness of that estate whereinto man fell, consisteth in the guilt of Adam’s first sin (Rom. 5:12, 19), the want of that righteousness wherein he was created, and the corruption of his nature, whereby he is utterly indisposed, disabled, and made opposite unto all that is spiritually good, and wholly inclined to all evil, and that continually (Rom. 3:10-19; Eph. 2:1-3; Rom. 5:6; Rom. 8:7-8; Gen. 6:5); which is commonly called Original Sin, and from which do proceed all actual transgressions (Jas. 1:14-15; Mat. 15:19).
Body of Divinity 4.2.
These names are given to original sin in Scripture:
It is called sin (Rom. 7:8).
The sinning sin (Rom. 7:13).
Sin that dwelleth in us (Rom. 7:20).
Sin that doth easily beset us (Heb. 13:1).
The body of sin (Rom. 7:23).
A law in the members, and the body of death (Rom. 7:24).
It is also called flesh (John 3:6; Rom. 7:5).
The old man (Rom. 6:6; Eph. 4:12; Col. 3:9).
The law of sin (Rom. 7:25).
The wisdom of the flesh (Rom. 8:6-7).
The law of sin and of death (Rom. 8:2).
The plague in ones own heart (1 Kings 8:38).
The root of bitterness (Heb. 12:15).
It is called by the Fathers “Original Sin.”
It is not a mere want of original righteousness, carentia justitiae originalis debitae inesse [lack of original righteousness being inherent]. The Papists make Adam fallen to be the man in the Gospel that was wounded as he was going to Jericho, by thieves, and lay half dead [Luke 10:30]. The scope of that parable is to teach who is to be accounted our neighbor. Our nature is not only void of God’s image, (Romans 3:12) but fertile of all evil (Gen. 6; Psa. 14 & 53; Acts 13:10; Eph. 4:19). It is hard to determine what kind of positiveness can be in sin. There are two kinds of privations: 1. Simple, which doth merely deprive, as darkness doth light. 2. Compound, which besides the mere privation includes the contrary form, privatio male disponens [privation and disposed to evil]; as sickness, besides the mere privation of health, includes the humours abounding. Health is affectus corporis ad actum benè agendum [affected by the body to act and do well], disease is the contrary.
We call it positive, because the Scripture describes it by habitual deprivation, “The heart is deceitful above all things, and desperately wicked: who can know it?” (Jer. 17:9). When we say such an one is a drunkard, it is not only a mere privation of sobriety, but a readiness to that sin, because of the inhesion of it, and to denote the efficacy of it. Original sin is an affection ad actum malè agendum [to act maliciously]. It is both a privation of the habit of original righteousness and also an evil disposition and proneness to all manner of sin infecting all the parts and faculties of the soul. 
Original Sin is:
1. An internal uncleanness.
“Unto the pure all things are pure: but unto them that are defiled and unbelieving is nothing pure; but even their mind and conscience is defiled” (Titus 1:15). It is called concupiscence [i.e. lust], which is the act of the will.
2. An abiding uncleanness.
It never ceaseth so long as a man liveth, to provoke him to sin, (Rom. 7:21). Actual sins are but transient acts, an affront to God’s commands; Original Sin is a rooted contrariety to his nature.
3. An abounding uncleanness.
(Psa. 14; Rom. 3). It defiles all men and all of men. In the first Covenant, Adam was made a root of all mankind, therefore all sinned and died in him, being all in his loins (Heb. 7:9). Hence all that descend from him are children of wrath (Eph. 2:3).
Sin came upon all by Adam two ways:
1. By imputation; The Lord in justice imputing the guilt of the first sin to all his posterity (Rom. 5:12, 14, 19; 1 Cor. 15:22, 45, 47).
2. By propagation; The lump and root of all mankind being corrupted, so are the branches (Gen. 5:3; Job 14:14; Rom. 11:16). They are dead in sins (Mat. 8:22; Luke 15:24; Eph. 2:1; 1 Tim. 5:6), under the power of sin naturally, under the guilt of sin legally, (Rom. 5:15, 18). Therefore regeneration is called a creation and resurrection (Rom. 6:5; John 3:35; Eph. 1:19-20).
1. All the faculties of the soul are dead, the mind blind (Zech. 11 ult.; 1 Cor. 2:14; Eph. 4:17) and vain in its apprehensions, resolutions, thoughts (Jer. 4:14). 2. Reasonings, The will most desperately shut against Christ and duty, (Mat. 15:29; 23:37; Luke 19:14; John 8:44) violently evil (Isa. 57:17). The memory retains toys and lets go solid things. The affections are not carried to their right objects; we love sin, are angry with those that reprove us, or not in a right measure, we over-love, over-joy (Col. 3:5), the affections are contrary one to another, and inconstant. The conscience is not active in accusing or excusing, (Titus 1:15; 1 Tim. 1:19; 4:2; Eph. 4:19). 3. They are dead in respect of spiritual duties (1 Thes. 1:3?; Heb. 1), the Sabbath is a burden. 4. In their profession, (Rev. 3:1; Jude 12). 5. In their whole conversation.
4. An active powerful uncleanness.
“But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members” (Rom. 7:23). It is peccatum actuosum [acting sin], though not actuale [in action], it acts continually, “every imagination of the thoughts of his heart was only evil continually” (Gen. 6:5). Sinful acts and habits both flow from the pravity of our nature (Mat. 7:16-18).
5. A diffusive or infectious uncleanness.
Like a leprosy or plague (Psa. 106:36) it makes all bad that we meddle with; to the defiled all things are defiled [Titus 1:15].
It may well be called the sinning sin, not only because it is the punishment of sin and the cause of sin, but because itself is sin (as Augustine [says]).
Next unto the sin against the Holy Ghost and contempt of the Gospel, original sin is the greatest sin (Mr. Shepherd).
All the sins of our lives are but original sin exercised and multiplied.
The will of man is more willful than the understanding blind. 
The seat or subject of this sin is the whole man. Some say only the passions, that we have sound reason and and free will, every faculty of the soul and member of the body is corrupted, but principally the soul (Eph. 4:18-24; Rom. 12:1-2; 1 Thes. 5:23), and in it: the understanding, will, and affections (Eph. 4:18). The will is primarium peccati subjectum [the primary subject of sin] (Rom. 7:14).
Faulty opinions about original sin examined
The Manichees and Illyricus, a Lutheran, make original corruption to be the essence and substance of a man; those places of Scripture where it is called the old man, a body of death, and the flesh, give no warrant for it, but the Scripture useth them 1. To shew how near it is to us, and inseparable, even as our hands and feet, and that we have it from our birth. 2. To teach us that in all repentance, and so in the graces of sanctification, the greatest matter lieth within.
The Pelagians out of hatred to this opinion ran too far into another extreme, holding that as man was born sine virtute [without virtue], so also sine vitio [without vice]; and they say that original sin is derived, not by propagation, but imitation and example. We are by nature (not imitation) the children of wrath [Eph. 2:3]. Pagans and Heathens never heard of Adam, and many sins are committed that Adam never did, and they imitate not him, the first drunkard and adulterer had no example.
The Ancient Fathers against the Pelagians, and the Orthodox against the Arminians hold that original sin is propagated from Adam to all his posterity. 1. God chargeth this on all the sons of men (Ez. 6; Isa. 48:4 c.f. verse 8), therefore it comes to them by natural inclination. 2. The saints who have studied their own spirits have confessed this to be in them (Psa. 51:5; Rom. 7:18). 3. Adam in his fallen condition must communicate such a nature as he had, viz. defiled, “Who can bring a clean thing out of an unclean? not one” (Job 14:4). 4. Adam infected nature, and after nature infected the person. The continual actings of the depravedness of our nature in our conversation (Psal. 58:3) and the misery that lay on all men by nature, even infants, prove this (Rom. 5:12) and the necessity of regeneration (John 3:5).
The faculties of the soul only (not the substance thereof) are corrupted, otherwise the soul could not be immortal, neither could Christ take our nature upon him. The substance of man abstractedly considered is God’s creature since the Fall and therefore good (1 Tim. 4:4). Regeneration restores not the substance of man but the qualities.
Dr. Ames saith that Grevinchovius denied original sin, and Dr. Twisse proves by this argument that the Arminians deny it. As many as teach that all the posterity of Adam have as much power to everything that is good as Adam in innocency, they deny original sin. But the Arminians teach that all the posterity of Adam have as much power to everything that is good as Adam had in the state of innocency, for they hold that all Adam’s posterity have such power to every good work, that they want no other help but the persuasion and the concourse of God, which Adam himself needed to every good work.
The Semi-Pelagians also, the Socinians and Anabaptists deny this original venom or blot to be a sin; the Anabaptists that they might wholly take away Paedobaptism denied original sin, that there might not be a cause why infants should be baptized.
The denying of this fundamental article of original sin is dangerous. What need then of the Gospel, what need of Christ himself, if our nature be not guilty, depraved, and corrupted? These are not things in quibus possimus dissentire salva pace ac charitate (Augustine) about which we may dissent without loss of peace or charity.
The Papists say
1. Original corruption hath not rationem peccati [liability to sin], but is only a privation of original righteousness. The Council of Trent decreeth it not to have the nature of sin. Bellarmine saith it is a simple thing to be humbled for original sin. Pighus saith it is no sin at all. Andraedeus, its the least of sin.
2. That the concupiscence and lust which riseth from the corruption of our nature, the motions unto evil that we feel in ourselves, are no sins (but are called so abusively or metonymically, because they are from, and incline to, sin) till we consent unto them and obey them, till they reign in us. 
When our divines urge that concupiscence is called sin several times in the sixth, seventh, and eighth chapters to the Romans, Bellarmine saith the Apostle doth not say it is peccatum proprie [sin properly speaking]. 
3. That original sin after baptism is done away. 
4. That the Virgin Mary was not conceived in sin. 
1. The Spirit of God in the holy Scripture expressly calleth the corruption of our nature sin, as Psalm 51:5 and in the sixth, seventh, and eight chapters of the Romans fourteen times at the least (c.f. Heb. 12:1, “the sin which doth so easily beset us“).
The Scripture saith expressly, our original corruption is the cause of all our actual sins, “But every man is tempted, when he is drawn away of his own lust, and enticed” (James 1:14); “…the corruption that is in the world through lust” (2 Peter 1:4).
Infants that are baptized, which have no other sin but original, and who never consented to it nor obeyed it in the lusts thereof, do die (Rom. 5:14), therefore it must needs be sin, and may be truly and properly so called; for sin is the only cause of death (Rom. 5:12).
Whatever holdeth not conformity with the rule of righteousness, the law of God, is sin, it hath the nature of sin in its irregularity and defect of good, and the effects of sin.
2. The Scripture expressly teacheth us, that this concupiscence even in the regenerate, these evil motions that rise in us, though we consent not unto them, though we resist them, are yet a swerving from the law of God and a breach of it (Luke 10:27), nay in the regenerate this corruption of our nature doth not only swerve from the law of God, but opposeth and resisteth the Spirit of God (Rom. 7:2; Gal. 5:17), therefore it must needs be sin. This argument convinced Paul’s conscience, “What shall we say then? Is the law sin? God forbid. Nay, I had not known sin, but by the law: for I had not known lust, except the law had said, Thou shalt not covet” (Rom. 7:7). He means those motions unto evil which the heart doth not delight in nor consent unto [Rom. 7:15].
When the Apostle saith, Rom. 6:12 “Let not sin reign in your mortal bodies.” By sin (saith their Cardinal Bellarmine) all men understand concupiscence, and Ribera on Heb. 12:1 saith that by sin the Apostle understandeth concupiscence, calling it so with an article τήν, that is the sin, a note of singularity. Cajetan in Rom. 7 calleth it formally a sin. 
The proper definition of sin being this, a transgression of God’s law, therefore concupiscence is sin, see Exodus 20:17.
Objection: The regenerate have no sin left in them.
“…there is no spot in thee” (Cant. 4:7); “He that is washed needeth not save to wash his feet, but is clean every whit: and ye are clean…” (John 13:10); “Then will I sprinkle clean water upon you, and ye shall be clean: from all your filthiness, and from all your idols, will I cleanse you” (Ezek. 36:25); “According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love:” (Eph. 1:4); Therefore the regenerate have no sin left in them.
The Church in this present world is said to be all fair, as it wholly shines with its Spouse’s beauty which it puts on. Concupiscence in respect of its own nature is a sin, but in respect of the person (who is a party regenerate in whom the guilt is pardoned), it is as no sin. When the Fathers say that lust is taken away in the regenerate, they understand according to the guilt, not the thing.
3. Original sin after baptism is not done away, children are perverse; death cannot seize where there is no sin. How comes it to pass that infants baptized die before they come to actual offending, if baptism have abolished in them their original stain?
4. The Virgin Mary was not conceived without original sin; in her song she rejoiceth in God her Saviour (Luke 1:47 and 2:22), Christ came to save that which was lost, (Mat. 18:11. See Job 14:4 & 1 Cor. 15:2; Rom. 5:12, 16 & 3:9: Gal. 3:22).
All the ancient Fathers, as far as we can learn out of their writings, believed that the blessed Virgin Mary was conceived in original sin. 
The Dominicans generally hold that she was conceived in sin.
All are infected with Adams sin:
1. The Heathens, Pagans, Infidels (Rom. 1:18-28), to the last. 2. The Jews (Rom. 2 latter end). 3. Christians (Rom. 3:9-19). 4. Infants, (Rom. 5:12-13). They are innocent in respect of actual transgression, not in respect of original pollution; are born blind, lame. 5. Children of believing parents.
All men are equally guilty of original sin:
1. In reference to Adam (Rom. 5:12, 14). 2. They are equally deprived of God’s image (Rom. 3:9, 11), reprobate to every good work. 3. Are equally depraved and corrupted (Rom. 3:12-14).
1. All men are equally in Adam, one was not more in his loins than another (Rom. 5:12, 19). 2. All men equally partake of the human nature, are men as much as other men (Isa. 8:7; Acts 17:28). 3. Total privations are equal, all men are spiritually dead.
Though the seed of all evil be in every man’s heart by nature, yet even among natural men, some are better or rather less wicked than others, as one weed is less noxious than another.
This corruption shews itself less because of restraining grace:
1. In moral and civil men, whose lives are void of gross offenses; as amongst the Gentiles, Cato, Aristides the just; among Christians, Paul unconverted, and the young man who said he had kept all the commandments of God from his youth up. 2. In such who reverence God and his ministry, as Herod was better than Ahab who hated Micaiah [1 Kings 22:8]. 3. In such as are loving and abhor all malice and quarreling, than the malicious who are like the devil (Mat. 9) to whom it is a torment not to vex and torture men. 4. Such as are of a true and plain heart. 5. Such as prefer the public good before their private.
Yet such (though comparatively good) are not good in a saving way:
1. Because their heart is not renewed all this while. 2. They are not for the powerful exercise of all duties. 3. They have not a zeal to reclaim others. 4. They understand not the enjoying of God in all his ordinances.
Yet 1. Their condemnation will be less. 2. God bestows more blessings on them. 3. They have more peace.
How is Original Sin Propagated?
How original sin is propagated is a much more difficult question. One that the early church struggled with such that many of them were led into the error of Traducianism in order to answer it (Leigh, ibid., p. 314), which by and large the Lutherans followed but the Reformed rejected. Augustine thought it was one of the most difficult questions, saying: “Nothing is more known than that original sin is traduced [i.e. propagated], and nothing more obscure than how it is traduced” (Leigh, ibid., p. 313).
Francis Turretin argues that original sin is propagated by three degrees: “(1) in the conception of the body from an unclean seed; (2) in the creation of the rational soul with the want of original righteousness; (3) in the constitution of the whole compound by the union of the soul with the body.” 
Due to man’s psychosomatic unity, original sin is an accidental quality of our sensitive/animal faculties, that reside in the body, as well as our rational faculties, that reside in the soul (c.f. WCF 6:2).
“By the very strict connection of soul and body in one person, the intimate sympathy (sympatheian) of both, the mutual appetite for each other and the nice balancing (rhopēn) by which they embrace most closely and affect each other, and as the body tends towards the soul as its perfection and good, so the soul tends towards the body as its own proper domicile and organ of its actions; and as the soul communicates its affections to the body, so the body communicates its dispositions to the soul, whence also results the diversity of inclinations in different men.” (ibid., p. 643). 
Edward Leigh (4.3) briefly explains how this works:
“When we say the soul by conjunction with the body is defiled with sin, we mean not that the body works upon the soul and so infects it, as pitch doth defile with the very touch: but that at the same instant at which God gives the spirit, puts it in the body, Adam’s disobedience is then imputed to the whole person, and so by consequent corruption of nature and inclination unto evil, the pain of sin by God’s just appointment follows.
“God is a Creator of the soul in respect of the substance, so it is pure; but he is also a Judge, and so he creates the soul not simply as a soul, but as the soul of one of the sons of Adam, in which respect he forsakes it touching his Image which was lost in Adam, and so it is deprived of original justice, whence followeth original sin.”
God does not create the human soul sinful, rather, since the Fall, he creates them neither pure nor impure, without the quality of original righteousness nor of original sin.
“Now in this way God cannot be considered the author, but the avenger of sin. He is the author of the union as his own work; but not of the sin (another’s fault). He unites the soul to the body to preserve the species; he joins the soul deprived of righteousness to a corrupt body for a punishment of sin. Nor is God the cause of the corruption, if in joining the soul to the body he carries out an established law of nature (from which man proceeds properly, but the sinner only accidentally).” (Turretin, ibid., p. 643)
 B. Down. of Justificat. l. 7. c. 7. Vide Hoornbeeck. Anti Socin. l. 3. c. 3. Sect. 1.
 See Mr Fenners Epist. Ded. to his Hidden Manna or Mystery of saving grace.
 See the Rhemists in their Annotations on Rom. 7:7 and James 1:15. Bellarmine, de statu peccati, c. 9. 10.
 De statu peccati, c. 8.
 Si quis asserit non tolli in baptismate totum id quod veram & propriam rationem peccati habet, anathema sit. [If anyone…asserts that the whole of that which has the true and proper nature of sin is not taken away; but says that it is only rased, or not imputed; let him be anathema.] Decret. 5. Sectionis Concil. Trid.
 Piè ac rectè existimatur B. virginem Mariam singulari Deo privilegio ab omni omnino peccato fuisse immunem. Bellarmine, de Amiss grat. & statu pecc. l. 4. c. 15.
 Vide Cassand. Consult. art. 2. Tit. de Concupisc. p. 4.
 Vide Rivet. de Patrum autoritate, c. 7; Daille, Of the right use of the Fathers, l. 2. c. 6.
 Institutes of Elenctic Theology IX.xii, vol. 1, pp. 640-643.
 c.f. “The body is an essential part of man. Problem: whether the true and substantial form of man is both thus united to the body and substantially actuates and informs it? Answer: in the affirmative, provided the union is not per accidens, but per se, not accidental, but substantial. Hence the soul is rightly called the essential form of man.” (Voetius, Selectarum Disputationum theologicarum I.757).