John Calvin Commentary Isaiah 65:4

"that sit among the graves, and lodge in the secret places; that eat swine`s flesh, and broth of abominable things is in their vessels;" — Isaiah 65:4 (ASV)

Who dwell in the graves. He enumerates other kinds of superstitions; and although, due to its brevity, the description is obscure, yet we may easily learn from other passages what their nature was. For as necromancy was generally practiced among pagan nations, the Jews also consulted demons “in graves and deserts,” instead of consulting God alone, which they should have done; and, as if they were seeking answers from the dead, they took pleasure in being deceived by the illusions of demons.

How solemnly the Lord had forbidden it appears very clearly from Deuteronomy 18:10-11, and other passages; and we have seen something of this kind in an earlier part of this book (Isaiah 8:19). In general, we are taught that God demands nothing more than obedience, which he prefers to slain beasts and sacrifices (1 Samuel 15:22).

Who eat swine’s flesh. Previously, he complained that the worship of God was polluted by strange inventions. Now he adds that they set aside every distinction, so that they do not distinguish between the clean and the unclean; and he brings forward a single instance: that they do not abstain from “swine’s flesh.”

But it may be thought that this was a small matter. Very far from it; for we should not judge from our own opinion, but from that of the legislator, how heinous a sin it is; and nothing that the Lord has forbidden should be considered trivial (Leviticus 11:7; Deuteronomy 14:8).

This related to the external profession of faith, by which the Jews were duty-bound to testify how widely they differed from the pollution of the Gentiles. Therefore, from that rule which the Lord enjoins on us, we must not swerve even a hair’s breadth.