Now they will be brought into a renewed covenant that operates on a more profound level, just as circumcision of the heart is more profound than outward circumcision. They have repented, but beyond repentance there is transformation. This is the work of God through Messiah. Circumcision of the heart, as Ramban points out later in his discussion, speaks of the renewed covenant that Jeremiah and Ezekiel describe. This transformation is to take place in the days of Messiah.
Circumcision can only take place after a birth.
Coming eight days after birth, it is in a sense a completion of the birth process. Only then is the child given his name, because now the process of birth is complete. Beyond repentance there is transformation, a transformation so thorough that it will render us new men and women. Torah calls us to return to its teachings and walk in its ways, but it also contains the promise of a renewed covenant and a new birth. It is up to us to repent, but God responds to our repentance.