On The Holy Trinity by St. Augustine
Chapter 9.--We Say that Future Blessedness is Truly Eternal, Not Through Human Reasonings, But by the Help of Faith. The Immortality of Blessedness Becomes Credible from the Incarnation of the Son of God.
12. Whether human nature can receive this, which yet it confesses to be desirable, is no small question. But if faith be present, which is in those to whom Jesus has given power to become the sons of God, then there is no question. Assuredly, of those who endeavor to discover it from human reasonings, scarcely a few, and they endued with great abilities, and abounding in leisure, and learned with the most subtle learning, have been able to attain to the investigation of the immortality of the soul alone. And even for the soul they have not found a blessed life that is stable, that is, true; since they have said that it returns to the miseries of this life even after blessedness. And they among them who are ashamed of this opinion, and have thought that the purified soul is to be placed in eternal happiness without a body, hold such opinions concerning the past eternity of the world, as to confute this opinion of theirs concerning the soul; a thing which here it is too long to demonstrate; but it has been, as I think, sufficiently explained by us in the twelfth book of the City of God. But that faith promises, not by human reasoning, but by divine authority, that the whole man, who certainly consists of soul and body, shall be immortal, and on this account truly blessed. And so, when it had been said in the Gospel, that Jesus has given |power to become the sons of God to them who received Him;| and what it is to have received Him had been shortly explained by saying, |To them that believe on His name;| and it was further added in what way they are to become sons of God, viz., |Which were born not of blood, nor of the will of the flesh, nor of the will of man, but of God;| -- lest that infirmity of men which we all see and bear should despair of attaining so great excellence, it is added in the same place, |And the Word was made flesh, and dwelt among us;| that, on the contrary, men might be convinced of that which seemed incredible. For if He who is by nature the Son of God was made the Son of man through mercy for the sake of the sons of men, -- for this is what is meant by |The Word was made flesh, and dwelt among us| men, -- how much more credible is it that the sons of men by nature should be made the sons of God by the grace of God, and should dwell in God, in whom alone and from whom alone the blessed can be made partakers of that immortality; of which that we might be convinced, the Son of God was made partaker of our mortality?