“For educated men distinguish the end of the work from the end of working”
(Samuel Rutherford, Disputatio Scholastica de Divina Providentia, p. 215). 
This is a basic introduction to a distinction that one will commonly come across in reading Reformed Theology. This distinction will greatly assist in one’s understanding of theological concepts. We will define this distinction and then give common examples of it. It is important to be able to understand this distinction so that we can better understand what God would have us believe about Him, and what duty He requires of us, as written in the Word of God (WLC Q. 5).
Finis Operis vs. Finis Operantis
The Latin term finis means end or goal, and refers to the final cause of something, “the ultimate purpose for which a thing is made or an act is performed.” The term operis means work or action. The term operantis means working or acting, referring to the one performing the action. 
Finis operis, the end of the act, is the inherent goal or natural result of an act in itself. The end is so intrinsic to the act that it cannot be done without the end necessarily following it.
Finis operantis, the end of acting, is the willed intention of the person who acts.
Franciscus Junius uses different terms, the natural end and the willed end, to explain the same concept:
“Nature very plainly teaches this order of distinguishing ends. For in all situations and actions something both of nature and of will always is maintained, such that it is aimed toward some end. Nature resolutely places this one end before itself; will selects it in a mutable fashion. Now indeed, of those things which are accomplished only by the instinct of nature apart from some motion of the will, the reasoning is such that these never stray or are in and of themselves alienated from the end that nature has established. They customarily are aimed toward several ends, according to the principles by which they are governed. So it happens that individual things arising from nature pursue a natural end, while those arising from will pursue a willed end, whether one that has been carefully considered or one that has not. They pursue the considered end by a proper ordering of nature and will acting in harmony. But they pursue the unconsidered end when an improper confusion of these has occurred.” 
Let’s consider a few examples of how this distinction is used in various theological topics.
The External Call of the Gospel
The inherent goal of the call to sinners to repent and believe is their salvation, however the purpose of God in the gospel being preached to every creature is not the salvation of each individual creature, rather, only the elect will respond in faith to the external call of the gospel.
“The end of [external] calling can be considered in two ways: either on the part of God or on the part of the thing (which is called the end of the worker [finis operantis] and the end of the work [finis operis]). Although each is conjoined in the elect, yet in others they are separated…in the gospel call, the end of the thing is the salvation of man because by its nature it tends to the bringing of him to salvation by faith and repentance; but not at once with respect to all the called is it the end of God, but only of those to whom he decreed to give faith and salvation.”
Francis Turretin, Institutes of Elenctic Theology, XV.ii.iv., vol. 2, p. 504.
“An action is constituted evil three ways—by proceeding from a wrong principle, by being directed to a wrong end, and by being done in a wrong manner” (Girolamo Zanchi, Observations on the Divine Attributes, 4.3).
Morality is rightly evaluated in terms of the end of the act itself and the end of the moral agent in the act. The “ultimate end,” says Turretin, “in morals holds the relation of the most universal principle (on which the rest [of all moral things] depend)” (Institutes, IX.viii.vi, vol. 1, p. 612). Some actions are contrary to the end for which they exist teleologically.
“The end of the act itself is essentially disordered, although the agent can have a good intent, which is not enough to excuse the act, as is the case when intending to steal in order to give. (Finis etiam ipsius actus secundum suam naturam est inordinatus, licet ex intentione agentis possit esse finis bonus, qui non sufficit ad excusationem actus, ut patet in eo qui furatur intendens eleemosynam dare.)”
Thomas Aquinas, De Malo, Q.15, A.1.
In the same section Aquinas makes the distinction between disordered internal desires and disordered external acts. He explains that every act is “disordered of its very self…that is not properly related to its requisite end.” 
There are intentions in acts considered in themselves, and intentions of rational persons. Actions can be sinful in and of themselves even if the intentions are good, and likewise actions (considered in themselves) can be good even if the intentions are evil. Zacharius Ursinus notes that truly good works, although imperfect, are done 1) in faith, 2) according to the command of God, and 3) to the glory of God; the works themselves are good, and the intention of the one performing them is good. The end of the act and the end of the actor are consistent in truly good works. Yet works are apparently good, but not truly good, when the external action is in accordance with God’s Law, but the internal intentions are not in faith nor for the glory of God; such superficial good works are “commanded by God, and are in their own nature good, but become evil by an accident, not being done in the manner, nor with the end with which they ought to be performed.” (Commentary on the Heidelberg Catechism, on Q. 91, p. 479). 
For instance, adultery is sinful even if one thinks he is doing it for a noble reason (e.g. Abraham and Hagar (Gen. 16:2)). Or if one is singing psalms but not with grace in the heart (Col. 3:16), one’s external actions are not sinful, but his intentions are. Russian Roulette in itself is wrong even when the people playing aren’t intending on being the one to get the bullet. Bulimia is also contrary to nature, etc. We have to consider human intentions as well as the natural effect of things in themselves.
Sodomy and other unnatural sex acts are immoral in themselves because it is “leaving the natural use of the woman” (Rom. 1) and is to desire “strange flesh” (Jude 7) which Scripture also calls “vile affections” (Rom. 1:26) or being “without natural affection” (Rom. 1:31; 2 Tim. 3:3). Even if one were so desensitized as to intend these unspeakable acts in a loving way, the end of these acts militate against all three ends of marriage, which are, 1) the unity and mutual help of husband and wife (Gen. 2:18), 2) the increase of mankind (Gen. 1:28) and of the Church with an holy seed (Mal. 2:15), i.e. procreation and child rearing, and 3) preventing of uncleanness (1 Cor. 7:2&9) (WCF 24:2).
In the same way, the finis operis, end of the act, of contraception unnaturally agitates against one of the ends of marital intimacy, namely, procreation (Gen. 1:28; Mal. 2:15) and is therefore sinful (Gen. 38:7-10). (cf. Contraception pt. 4: Onanism).
This is also why lying is always wrong, because speech is naturally ordered to truth. Evil must not be done that good may come (Rom. 3:8). “What is evil according to itself in its own kind can in no way be made good and lawful. Now every lie is evil in itself, falling under undue matter. It is a disorder both of the liar in himself because it is against the order of nature of his speech (the interpreter of the mind plainly disagrees with the mind) and against his neighbor because each one owes a regard for the truth to his neighbor, from natural obligation and that of divine right.” (Francis Turretin, Institutes of Elenctic Theology, XI.xx.v., vol. 2, p. 130).
Intended Subjects of the Atonement
“An intention or a conscious goal is that which an agent aims to accomplish and the means are that which is used for attaining the intention. When the agent acts according to its nature, then the end of the action and the end of the agent is one and the same. But when the means are not fitted for the intentional end, then a distinction must be inferred between the end of the action and the end of the agent; between the intention and the intender. Now, the doctrine of divine simplicity implies that in God intention and Intender—act and Agent—cannot be other than one and the same. In humans intention and intender—acts and agents—may not be one and the same. In God the means for attaining the salvation of the elect are not, indeed cannot, be disproportionate to that end. There cannot be conditions—conditional redemption—to God. For according to the doctrine of divine simplicity, each thing is related to God, but God is not (reciprocally) related to anything. Yet, universalism anthropomorphically pictures God as using means that are not proportionate to the end and assumes that there is one intention in some salvific act and another intention in some other salvific act. Particularism upholds the doctrine of simplicity and consistently maintains the otherness of God in intending to save humans in Christ. Every salvific action of God is particular in intention, since in God intention and Intender cannot be other than one and the same.”
Sebastian Rehnman, A Particular Defence of Particularism, Volume 6, Issue 1,
The Ends of the Ministry vs. the Ends of the Magistracy
“The supreme end of Magistracy is only the glory of God, as King of Nations, and as exercising dominion over the inhabitants of the earth: And in that respect the Magistrate is appointed to keep his Subjects within the bounds of external obedience to the moral Law, the obligation where of lies upon all Nations, and all men. The supreme end of the Ecclesiastical power, is either proximus [near] or remotus [remote]. The nearest and immediate end is the glory of Jesus Christ, as Mediator and King of the Church. The more remote end is the glory of God, as having all power and authority in heaven and earth….if the Magistrate be Christian, it is incumbent to him so to administer that high and eminent vocation of his, that Christ may be glorified as King of the Church, and that this Kingdom of Christ may flourish in his Dominions, (which would God every Magistrate called Christian did really intend.) So then the glory of Christ as Mediator and King of the Church, is to the Ministry both finis operis, and finis operantis. To the Magistrate, though Christian, it is only finis operantis; That is, it is the end of the godly Magistrate, but not the end of Magistracy: whereas it is not only the end of the godly Minister, but the end of the Ministry itself. The Minister’s intendment of this end, flows from the nature of their particular vocation. The Magistrate’s intendment of the same end, flows from the nature of their general vocation of Christianity, acting, guiding, and having influence into their particular vocation.”
 Richard Muller, Dictionary of Latin and Greek Theological Terms, p. 61.
 The terms agentis (agent) and actus (act) are also frequently used.
 Franciscus Junius, A Treatise on True Theology, p. 206. (Junius goes on to explain that in the following discourse he is speaking of the natural end of theology specifically, hence he doesn’t provide us with an example for our purposes in this article).
 cf. Thomas Aquinas, Summa Theologiae, I-II, Q. 18, A. 6.
 cf. Turretin, Institutes of Elenctic Theology, XVII.iv.i-viii, vol. 2, pp. 706-708. on good works.