Deuteronomy 30:6

6.And the Lord thy God will circumcise thine heart. This promise far surpasses all the others, and properly refers to the new Covenant, for thus it is interpreted by Jeremiah, who introduces God thus speaking, —

“Behold, the days come that I will make a new covenant with the house of Israel, and the house of Judah, not according to the covenant that I made with their fathers, which covenant they brake, but I will put my law in their inward parts, and write it in their hearts.” (Jeremiah 31:31.)

Moses now declares the same thing in different words, that, lest the Israelites, according to their wonted instability, should fall back from time to time into new rebellions, a divine remedy was needed, i. e. , that God should renew and mould their hearts. In short, he reminds them that this would be the chief advantage of their reconciliation, that God should endow them with the Spirit of regeneration. There is a metaphor in this word circumcise; for Moses alludes to the legal sign of consecration, whereby they were initiated into the service of God. The expression, therefore, is equivalent to his saying, God will create you spiritually to be new men, so that, cleansed from the filth of the flesh and the world, and separated from the unclean nations, you should serve Him in purity. Meanwhile, he shews that, whatever God offers us in the Sacraments, depends on the secret operation of His Spirit. Circumcision was then the Sacrament of repentance and renewal, as Baptism is now to us; but “the letter,” as Paul calls it, (Romans 2:27,) was useless in itself, as also now many are baptized to no profit. So far, then, is God from resigning the grace of His Spirit to the Sacraments, that all their efficacy and utility is lodged in the Spirit alone.

Although Moses seems to make a division of the matter between men and God, so as to ascribe to them the beginning of repentance, and to make Him the author of perseverance (only, (285)) nevertheless this difficulty is easily solved; for according to the ordinary manner of Scripture, when he exhorts them to repentance, he is not teaching them that it is a gift of the Spirit, but simply reminding them of their duty. Meanwhile, the defenders of free-will foolishly conclude, that more is not required of men than they are able to perform; for in other places they are taught to ask of God whatever He enjoins. Thus, in this passage, Moses treats of the means of propitiating God, viz., by returning into the right way with an unfeigned heart; but, after he has testified that God will be gracious to them, he adds, that there is need of a better remedy, so that, being once restored by Him, they may be perpetual recipients of His grace. Still, it is not his intention to restrict the circumcision of the heart to the subsequent course of their lives, as if it depended on their own will and choice to circumcise themselves before God should work in them. And surely it is not at all more easy to rise when you have fallen, than to stand upright after God has set you up. I confess that perseverance is an excellent grace; but how shall the sinner, who is enthralled to Satan, free himself from those chains, unless God shall deliver him? Therefore, what Moses lays down as to the gift, of perseverance, applies no less to the commencement of conversion; but he only wishes to teach us that, although God should pardon our sins, that blessing would be but transient, unless He should keep us in subjection to His Law. And, in fact, He regenerates by His Spirit unto righteousness all those whose sins He pardons.