" God requires integrity and holiness of life; he indicated by the symbol how this could be attained, that is, by cutting off in man whatever is born of the flesh, for his whole nature had become vicious. He therefore reminded Abraham by the external sign, that he was spiritually to cut off the corruption of the flesh; and to this Moses has also alluded in De 10:16. And to show that it was not the work of man, but of God, he commanded tender infants to be circumcised, who, on account of their age, could not have performed such a command. Moses has indeed expressly mentioned spiritual circumcision as the work of divine power, as you will find in De 30:6, where he says, “The Lord will circumcise thine heart:” and the Prophets afterwards declared the same thing much more clearly.

As there are two points in baptism now, so there were formerly in circumcision; for it was a symbol of a new life, and also of the remission of sins. But the fact as to Abraham himself, that righteousness preceded circumcision, is not always the case in sacraments, as it is evident from the case of Isaac and his posterity: but God intended to give such an instance once at the beginning, that no one might ascribe salvation to external signs. "

― John Calvin