Isaiah 66:17

17.They who sanctify themselves. He now describes those enemies of whom he said, that God’s anger would be kindled against them; for it might have been doubtful whether he spoke of foreign and avowed enemies, or directed his discourse to the despisers of God, although they had been mixed with those who were elect and holy; and therefore he plainly addresses the false and degenerate Jews. Nor have I any doubt that, in the first place, he rebukes hypocrites, and, in the second place, when he says, “Who eat swine’s flesh,” he describes men of immoral lives, that is, those who were openly wicked and grossly licentious. Hypocrites sanctified themselves, that is, assumed false disguises of holiness, and deceived many under this pretense.

They purified themselves in the gardens; that is, they polluted themselves with various superstitions, although they imagined that, by means of those superstitions, they rendered themselves pure in the sight of God. Others, without any reserve, despised God and all religion. It is therefore a general statement, in which he includes all the ungodly, to whatever class they may belong; that is, both those who openly display their wickedness, and those who hide and cover it by various disguises.

Behind one in the midst. (227) Some commentators supply the word “pool,” or “laver;” as if holy water had been placed “in the midst” of the garden for ablutions. But another meaning would be equally appropriate; that every one chose a God for himself exclusively, and therefore every one out of many trees had his own tree.