59 So, then, having obtained everything, let us know that we are born again, but let us not say, How are we born again? Have we entered a second time into our mother's womb and been born again? I do not recognize here the course of nature. But here there is no order of nature, where is the excellence of grace. And again, it is not always the course of nature which brings about conception, for we confess that Christ the Lord was conceived of a Virgin, and reject the order of nature. For Mary conceived not of man, but was with child of the Holy Spirit, as Matthew says: “She was found with child of the Holy Spirit.” If, then, the Holy Spirit coming down upon the Virgin wrought the conception, and effected the work of generation, surely we must not doubt but that, coming down upon the Font, or upon those who receive Baptism, He effects the reality of the new birth.
Source: On the Mysteries (New Advent)