St. Gregory of Nazianzus: "Begotten of the Father"

"BELIEVE that the Son of God, the Eternal Word, Who was begotten of the Father before all time and without body, was in these latter days for your sake made also Son of Man, born of the Virgin Mary ineffably and stainlessly (for nothing can be stained where God is, and by which salvation comes), in His own Person at once entire Man and perfect God."

~St. Gregory of Nazianzus (c. 325 - 389): Orations, 40, 45.

Nativity, by Domenico Ghirlandaio. 

Tempera on panel, c. 1492; Fitzwilliam Museum, Cambridge.

St. Augustine: "The hope of His love"

"THE grace of God could not have been more graciously commended to us than thus, that the only Son of God, remaining unchangeable in Himself, should assume humanity, and should give us the hope of His love, by means of the mediation of a human nature, through which we, from the condition of men, might come to Him who was so far off,—the immortal from the mortal; the unchangeable from the changeable; the just from the unjust; the blessed from the wretched. And, as He had given us a natural instinct to desire blessedness and immortality, He Himself continuing to be blessed, but assuming mortality, by enduring what we fear, taught us to despise it, that what we long for He might bestow upon us."

~St. Augustine: The City of God, Book X, Chap. 29.

St. John Cassian: The Incarnation

"But in all the holy Scriptures He joins together and as it were incorporates in the Godhead, the Lord's manhood, so that no one can sever man from God in time, nor God from man at His Passion. For if you regard Him in time, you will find that the Son of man is ever with the Son of God. If you take note of His Passion, you will find that the Son of God is ever with the Son of man, and that Christ the Son of man and the Son of God is so one and indivisible, that, in the language of holy Scripture, the man cannot be severed in time from God, nor God from man at His Passion....There was not then before the birth of a Virgin the same eternity belonging in the past to the manhood as to the Divinity, but because Divinity was united to manhood in the womb of the Virgin, it follows that when we use the name of Christ one cannot be spoken of without the other."

~St. John Cassian (c. 360-c. 435): On the Incarnation of the Lord, Against Nestorius, Bk. 6:22.