11 Herein my soul, trembling and distressed, found a hope wider than it had imagined. First came its introduction to the knowledge of God the Father. Then it learned that the eternity and infinity and beauty which, by the light of natural reason, it had attributed to its Creator belonged also to God the Only-begotten. It did not disperse its faith among a plurality of deities, for it heard that He is God of God; nor did it fall into the error of attributing a difference of nature to this God of God, for it learned that He is full of grace and truth.
Nor yet did my soul perceive anything contrary to reason in God of God, since He was revealed as having been in the beginning God with God. It saw that there are very few who attain to the knowledge of this saving faith, though its reward be great, for even His own received Him not though they who receive Him are promoted to be sons of God by a birth, not of the flesh but of faith. It learned also that this sonship to God is not a compulsion but a possibility, for, while the Divine gift is offered to all, it is no heredity inevitably imprinted but a prize awarded to willing choice.
And lest this very truth that whosoever will may become a son of God should stagger the weakness of our faith (for most we desire, but least expect, that which from its very greatness we find it hard to hope for), God the Word became flesh, that through His Incarnation our flesh might attain to union with God the Word. And lest we should think that this incarnate Word was some other than God the Word, or that His flesh was of a body different from ours, He dwelt among us that by His dwelling He might be known as the indwelling God, and, by His dwelling among us, known as God incarnate in no other flesh than our own, and moreover, though He had condescended to take our flesh, not destitute of His own attributes; for He, the Only-begotten of the Father, full of grace and truth, is fully possessed of His own attributes and truly endowed with ours.
Source: On the Trinity (New Advent)