Calvin offers these helpful insights in his comments on 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.” (Jer. 31:31–33.) 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, (Rom. 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.
Calvin, J. & Bingham, C.W., 2010. Commentaries on the Four Last Books of Moses Arranged in the Form of a Harmony, Bellingham, WA: Logos Bible Software. (Emphasis added.)