You shall not make for yourself any image nor any resemblance of the things which are in the heaven above, or on the earth here below, or in the waters which are under the earth. You shall not bow your head to them nor shall you honor them.
Just as by the previous commandment He declared Himself to be the only God, so now He states who He is and how He must be served and honored.
He forbids, then, that we should have in mind any resemblance to Him. He gives the reason for this in Deuteronomy 4:15-19 and in Isaiah 40:18-26, that is, because the Spirit is nothing like the body.
15 “So watch yourselves carefully, since you did not see any form on the day the Lord spoke to you at Horeb from the midst of the fire, 16 so that you do not act corruptly and make a graven image for yourselves in the form of any figure, the likeness of male or female, 17 the likeness of any animal that is on the earth, the likeness of any winged bird that flies in the sky, 18 the likeness of anything that creeps on the ground, the likeness of any fish that is in the water below the earth. 19 And beware not to lift up your eyes to heaven and see the sun and the moon and the stars, all the host of heaven, and be drawn away and worship them and serve them, those which the Lord your God has allotted to all the peoples under the whole heaven. Deuteronomy 4:15-19
18 To whom then will you liken God?
Or what likeness will you compare with Him?
19 As for the idol, a craftsman casts it,
A goldsmith plates it with gold,
And a silversmith fashions chains of silver.
20 He who is too impoverished for such an offering
Selects a tree that does not rot;
He seeks out for himself a skillful craftsman
To prepare an idol that will not totter.
21 Do you not know? Have you not heard?
Has it not been declared to you from the beginning?
Have you not understood from the foundations of the earth?
22 It is He who sits above the circle of the earth,
And its inhabitants are like grasshoppers,
Who stretches out the heavens like a curtain
And spreads them out like a tent to dwell in.
23 He it is who reduces rulers to nothing,
Who makes the judges of the earth meaningless.
24 Scarcely have they been planted,
Scarcely have they been sown,
Scarcely has their stock taken root in the earth,
But He merely blows on them, and they wither,
And the storm carries them away like stubble.
25 “To whom then will you liken Me
That I would be his equal?” says the Holy One.
26 Lift up your eyes on high
And see who has created these stars,
The One who leads forth their host by number,
He calls them all by name;
Because of the greatness of His might and the strength of His power,
Not one of them is missing. Isaiah 40:18-26
Furthermore, He forbids in religion that we should honor any image.
So let us learn from this commandment that the service and honor of God are spiritual, for, as He is Spirit, it follows that He desires to be served and honored in spirit and in truth (John 4:24).
23 But an hour is coming, and now is, when the true worshipers will worship the Father in spirit and truth; for such people the Father seeks to be His worshipers. 24 God is spirit, and those who worship Him must worship in spirit and truth.” John 4:23, 24
He then adds a terrible threat by which he declares how very seriously the transgression of this commandment offends Him: For I am the Lord, your God, powerful, jealous, visiting the iniquity of the fathers on the children until the third and fourth generation in those who hate me, and extending mercy to a thousand generations towards those who love me and keep my commandments.
It is as if He were saying that He is the only one to whom we must cling and that He cannot tolerate any companion god; and even that He will avenge His majesty and His glory if anyone transfers them to images or anything else; and that not only once, but on fathers, children and nephews, that is, on all — so many as there shall be — who will imitate the ungodliness of their fathers. In the same way He is saying that He will show His mercy and kindness to those who love Him and keep His Law. And He declares to us the grandeur of His mercy in that He extends it to a thousand generations, while He assigns only four generations to His vengeance.
John Calvin, Truth For All Time, pp. 12, 13