93 It requires closer examination to see the meaning of the first action of Moses in burning the calf in fire, and grinding it to powder, and sprinkling it in the water for the people to drink. The tables given to him, written with the finger of God, that is, by the agency of the Holy Spirit, he may have broken, because he judged the people unworthy of having them read to them; and he may have burned the calf, and ground it, and scattered it so as to be carried away by the water, in order to let nothing of it remain among the people.
But why should he have made them drink it? Every one must feel anxious to discover the typical significance of this action. Pursuing the inquiry, we may find that in the calf there was an embodiment of the devil, as there is in men of all nations who have the devil as their head or leader in their impious rites. The calf is gold, because there is a semblance of wisdom in the institution of idolatrous worship. Of this the apostle says, "Knowing God, they glorified Him not as God, nor were thankful; but they became vain in their imaginations, and their foolish heart was darkened.
Professing themselves to be wise they became foolish, and changed the glory of the incorruptible God into the likeness of corruptible man, and of birds, and of four-footed beasts, and of creeping things." From this so-called wisdom came the golden calf, which was one of the forms of idolatry among the chief men and professed sages of Egypt. The calf, then, represents every body or society of Gentile idolaters. This impious society the Lord Christ burns with that fire of which He says in the Gospel, "I have come to send fire on the earth;" for, as there is nothing hid from His heat, when the Gentiles believe in Him they lose the form of the devil in the fire of divine influence.
Then all the body is ground, that is, after the dissolution of the combination in the membership of iniquity comes humiliation under the word of truth. Then the dust is sprinkled in the water, that the Israelites, that is, the preachers of the gospel, may in baptism admit those formerly idolaters into their own body, that is, the body of Christ. To Peter, who was one of those Israelites, it was said of the Gentiles, "Kill, and eat." To kill and eat is much the same as to grind and drink. So this calf, by the fire of zeal, and the keen penetration of the word, and the water of baptism, was swallowed up by the people, instead of their being swallowed up by it.
Source: Reply to Faustus the Manichaean (New Advent)