The first commandment teaches who the true God is and that He alone is to be worshiped. The second commandment teaches that God alone prescribes how He wants to be worshiped. To worship God however we would like is the very definition of idolatry.
“Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth. Thou shalt not bow down thyself to them, nor serve them: for I the Lord thy God am a jealous God…” (Ex. 20:4-5).
1) There are two parts in the second commandment: do not make an image of God (v. 4), and do not worship God with the use of images (v. 5).
“There are two parts in the Commandment — the first forbids the erection of a graven image, or any likeness; the second prohibits the transferring of the worship which God claims for Himself alone, to any of these phantoms or delusive shows. Therefore, to devise any image of God, is in itself impious; because by this corruption His Majesty is adulterated, and He is figured to be other than He is. There is no need of refuting the foolish fancy of some, that all sculptures and pictures are here condemned by Moses, for he had no other object than to rescue God’s glory from all the imaginations which tend to corrupt it. And assuredly it is a most gross indecency to make God like a stock or a stone… I do not deny that these things are to be taken connectedly, since superstitious worship is hardly ever separated from the preceding error; for as soon as any one has permitted himself to devise an image of God, he immediately falls into false worship. And surely whosoever reverently and soberly feels and thinks about God Himself, is far from this absurdity; nor does any desire or presumption to metamorphose God ever creep in, except when coarse and carnal imaginations occupy our minds. Hence it comes to pass, that those, who frame for themselves gods of corruptible materials, superstitiously adore the work of their own hands. I will then readily allow these two things, which are inseparable, to be joined together; only let us recollect that God is insulted, not only when His worship is transferred to idols, but when we try to represent Him by any outward similitude.”
John Calvin, Commentary on Exodus 20:4
“Ye shall make you no idols nor graven image, neither rear you up a standing image, neither shall ye set up any image of stone in your land, to bow down unto it: for I am the Lord your God” (Lev. 26:1; c.f. Rom. 1:23, Deut. 4:12-19).
2) There is some truth to the connection between making a visual depiction of God and worshiping God by it. However, it is erroneous to suggest that it is possible to use images of God while not worshiping Him by them. That is, when the first part of the commandment is broken, the second is broken as well. An image is a representation of God “formed by the art and imagination of man” (Acts 17:29) and typically considered a medium through which we can esteem God or teach children about Him. The excuse that images of God are not being used for worship is necessarily untrue because we are to revere and worship God with all our heart, soul, and might (Deut. 6:5). If a picture of Christ does not “stir up devotion, it is in vain; if it stir up devotion, it is a worshiping by an image or picture, and so a palpable breach of the second commandment” (Vincent, A Family Instructional Guide).
“Pictures of Christ are in principle a violation of the second commandment. A picture of Christ, if it serves any useful purpose, must evoke some thought or feeling respecting him and, in view of what he is, this thought or feeling will be worshipful. We cannot avoid making the picture a medium of worship. But since the materials for this medium of worship are not derived from the only revelation we possess respecting Jesus, namely, Scripture, the worship is constrained by a creation of the human mind that has no revelatory warrant. This is will-worship. For the principle of the second commandment is that we are to worship God only in ways prescribed and authorized by him. It is a grievous sin to have worship constrained by a human figment, and that is what a picture of the Saviour involves.”
John Murray, Pictures of Christ.
3) God is Spirit and cannot be imaged, it is an ontological impossibility, and therefore sinful to attempt. “To whom then will ye liken God? or what likeness will ye compare unto him?” (Isa. 40:18). “Forasmuch then as we are the offspring of God, we ought not to think that the Godhead is like unto gold, or silver, or stone, graven by art and man’s device” (Acts 17:29). God has no parts and does not reveal Himself in parts because that is contrary to His nature and therefore impossible. By nature God is Spirit and cannot be seen. God has manifested Himself to people in various ways, but He has never been visibly seen, nor can He; but we have no right to fabricate a depiction of Him whether an Old Testament theophany, or the incarnation of the Son of God. None of the patriarchs, prophets, or apostles ever thought of visually depicting God after their encounters with Him. All images of Him require heretical presuppositions about His nature. So when people use images to teach their children about God, they are teaching them about a god “formed by the art and imagination of man” rather than the one true God of the Bible.
“The worship of God must be spiritual, in order that it may correspond with His nature…It is necessary, then, to remember what God is, lest we should form any gross or earthly ideas respecting Him. The words simply express that it is wrong for men to seek the presence of God in any visible image, because He cannot be represented to our eyes.”
John Calvin, Commentary on Exodus 20:4
To suggest that Christ’s humanity can be imaged presupposes a division between the divine and human natures of Christ (Nestorianism). “It is not lawful to have pictures of Jesus Christ, because his divine nature cannot be pictured at all; and because his body, as it is now glorified, cannot be pictured as it is” (Vincent, ibid.).
“Since God as Spirit is in essence invisible and immense, he cannot really be expressed by any art or image…Although Christ assumed human nature, yet he did not on that account assume it in order to provide a model for carvers and painters. He denied that he had come “to abolish the law and the prophets” (Matt. 5:17). But images are forbidden by the law and the prophets” (Deut. 4:15; Isa. 44:9). He denied that his bodily presence would be profitable for the Church, and promised that he would be near us by his Spirit forever (John 16:7). Who, therefore, would believe that a shadow or likeness of his body would contribute any benefit to the pious? (II Cor. 5:5). Since he abides in us by his Spirit, we are therefore the temple of God (I Cor. 3:16). But “what agreement has the temple of God with idols?” (II Cor. 6:16).”
2nd Helvetic Confession, chapter 4.
“I am the Lord: that is my name:
and my glory will I not give to another,
neither my praise to graven images”
Please listen to the following audio for more detail: R. Scott Clark, Images Of Christ Don’t Affirm His Humanity, They Deny It