The phrase which the Prophet uses is indeed harsh: he says, their heart goes after heart, so that some interpret this of imitation: namely, since God promises that he will be an avenger if any of the people conduct themselves after bad examples and unite in alliance with the wicked, just as if they glued together their hearts and affections, but that is harsh. The repetition is therefore superfluous, and the Prophet means nothing else than that God will be avenged if the Israelites follow their own heart, so as to walk in their own foulness and abominations. First of all we must understand the reason why the Prophet uses this sentiment. God had liberally poured out the treasures of his mercy, but since, hypocrites have always been mixed with the good, at the same time that they confidently boast themselves members of the Church, and use the name of God with great audacity; so that the Prophet uses this threat that they may not think all the promises which we hear of to belong to themselves promiscuously. For there were always many reprobate among the elect people, because not all who sprang from father Abraham were true Israelites. (Romans 9:6, 7.)

Since therefore it was so, the Prophet properly shows here that what he had previously promised was peculiar to God's elect, and to the true and lawful members of the Church, but not to the spurious, nor to the degenerate, nor to those who are unregenerated by the true and incorruptible seed. This is the Prophet's intention. But lest there might seem to be too much rigor when God, as it were, armed comes down into the midst to destroy all who do not repent, the Prophet here declares their crime -- namely, because their heart walks after their heart, that is, thine heart draws itself, and so the word heart is twice repeated. It is indeed a superfluous repetition but emphatic, when he says, that the heart of those who so pertinaciously adhere to their own superstitions is then impelled by its own self to new motions, so that by its continual tenor it goes always towards superstitions. Hence I will be an avenger, says God. Hence as often as God proposes to us testimonies of his favor, let each descend into himself and examine all his affections. But when any one lays hold of his own vices let him not please himself in them, but rather groan over them, and strive to renounce his own affections that he may follow God: neither let him harden himself in obstinacy, so that his heart may not proceed and rush continually towards evil, as is here said.