They made a calf. [1] Here he represents their rebellion as exceedingly base, in that they abandoned the true worship of God, and made to themselves a calf. For although it was their intention to worship God in this manner, yet the prophet reprehends their brutal stupidity, because they worshipped before the molten image, [2] and represented God by the figure of an ox which eateth grass [3] From this the prophet infers, that God had been robbed of his honor, and that all his glory had been tarnished. And surely it is so; for although the idolaters feign to serve God with great zeal, yet when, at the same time, they represent to themselves a God visible, they abandon the true God, and impiously make for themselves an idol. But he reproaches them with being guilty of still greater impiety, when he says, after the likeness of an ox that eateth grass; and contrasts with it their honor or glory. For seeing that God had clothed them with his own glory, what madness was it to substitute in place of him not only an ox, but the inanimate form of an ox, as if there were any resemblance between God who createth all kinds of food, and that stupid animal which feeds upon grass? It is necessary, however, to observe the design of the prophet, which is to point out the blindness of men as more base and abominable, because not contenting themselves with any common form of superstition, but casting off all sham they give themselves up to the most shocking forms of worshipping God. Had the people formed for themselves a likeness of God under the likeness of a man, even that would have been impiously robbing God of his due; how much more shameful was their conduct when they assimilated God to an ox? When men preserve their life by eating and drinking, they acknowledge how frail they are, because they derive from dead creatures the means of its continuation. How much greater is the dishonor done to God when he is compared to the brutal tribes? Moreover, the comparison referred to increases the enormity of their guilt. For what credit was it for a holy people to worship the inanimate likeness of an ox instead of the true God? But God had condescended to spread out the overshadowing wings of his glory upon the children of Abraham, that he might put on them the highest honor. Therefore, in denuding themselves of this honor, they had exposed their own baseness to the derision of all the nations of the earth. And hence Moses employs the phrase of nakedness, when he is showing that crime of idolatry: "And when Moses saw that the people were naked, (for Aaron had made them naked unto their shame among their enemies)" Exodus 32:25. Should any one be disposed to say that the ark of the covenant was a representation of God, my answer is, That that symbol was given to the children of Israel, not to engross the whole of their attention, but only for the purpose of assisting and directing them in the spiritual worship of God.

Footnotes

1 - This idol seems to have been an imitation of the Egyptian God Apis, or Serapis, a word which signifies the head of an ox, the Egyptians having exalted that animal to the rank of a god whom they absurdly worshipped, and to whom they resorted as to an oracle. "The modern Jews assert, that their ancestors were in that matter misled by certain Egyptian proselytes, who had accompanied the Israelites when they were delivered from their bondage. The Psalmist, it may be remarked, does not observe the order of time in his narrative, the making of the calf being prior to the fate of Dathan and Abiram. -- Comp. Exodus 32:4, 5." -- Cresswell.

2 - "More properly, the overlaid image;' or, more literally still, the metalline shell.'" -- Horsley. "The Hebrew word," says Mant, "here, as elsewhere, rendered by our translators molten image,' strictly and properly means the metalline case' or covering spread over' the carved wood. It is often joined with the carved wooden, image' which it covered. Aaron's calf was thus made of wood, and overlaid with gold."

3 - "That eateth hay -- the Egyptians, when they consulted Apis, presented a bottle of hay or of grass, and if the ox received it, they expected good success." -- Cresswell.

They made a calf in Horeb - Exodus 32:4. Probably in resemblance of the Egyptian god "Apis." The image was made by Aaron out of materials furnished by the people, and at their request Exodus 32:1-3, so that it might be said to be the act of the people.
And worshipped the molten image - The word rendered "molten" is from a verb נסך nâsak - to pour, to pour out; hence, to cast, to found; and it means anything that is made by fusion or casting. This image was cast Exodus 32:4, and hence, this name is given to it.

They made a calf in Horeb,.... A golden one, of the earrings of gold which were in the ears of their wives, sons, and daughters; these they took and melted down, and cast into the figure of a calf, or an ox, as it is afterwards said, in imitation of the Egyptians, who worshipped the ox; a people that had used them ill, from whose bondage they were just delivered; on whom they had seen the judgments of God inflicted, and who were notoriously wicked and an idolatrous people, and yet these they imitated; and, which was still a greater aggravation, this they did in Horeb, the mountain in which God appeared in so terrible a manner, with thunder and lightnings, and in fire, out of which he spoke to them; and when he gave them a body of laws, among which were one that forbid the worshipping of graven images; and yet they made and worshipped one in this very place, from whence they received this law; as follows.
And worshipped the molten image; when fashioned with a graving tool, and made a molten calf, they said, these be thy gods, O Israel, which brought thee out of Egypt; and they brought their burnt offerings and peace offerings, and ate and drank before it, and danced about it; all which were acts of idolatrous worship, Exodus 32:1. This was so heinous a sin, that the Jews say it is not expiated to this day, and that there is no punishment comes upon them but there is an ounce of the golden calf in it (q).
(q) T. Hieros. Taanioth, fol. 80. 4.

From indirect setting God at naught, they pass to direct.
made--though prohibited in Exodus 20:4-5 to make a likeness, even of the true God.
calf--called so in contempt. They would have made an ox or bull, but their idol turned out but a calf; an imitation of the divine symbols, the cherubim; or of the sacred bull of Egyptian idolatry. The idolatry was more sinful in view of their recent experience of God's power in Egypt and His wonders at Sinai (Exodus 32:1-6). Though intending to worship Jehovah under the symbol of the calf, yet as this was incompatible with His nature (Deuteronomy 4:15-17), they in reality gave up Him, and so were given up by Him. Instead of the Lord of heaven, they had as their glory the image of an ox that does nothing but eat grass.

A calf - When they were but just brought out of Egypt by such wonders, and had seen the plagues of God upon the Egyptian idolaters, and when the law of God was but newly delivered to them in such a tremendous manner.