For there is but one Son of God by nature, who in His compassion became Son of man for our sakes, that we, by nature sons of men, might by grace become through Him sons of God. For he, abiding unchangeable, took upon Him our nature, that thereby he might take us to Himself; and, holding fast His own divinity, he became partaker of our infirmity, that we, being changed into some better thing, might, by participating in His righteousness and immortality, lose our own properties of sin and mortality, and preserve whatever good quality he had implanted in our nature, perfected now by sharing in the goodness of His nature. For as by the sin of one man [Adam] we have fallen into a misery so deplorable, so by the righteousness of one Man, who also is God, shall we come to a blessedness inconceivably exalted.
City of God, book 11, chapter 15.