25 For Christ had indeed a body, but unique, as befitted His origin. He did not come into existence through the passions incident to human conception: He came into the form of our body by an act of His own power. He bore our collective humanity in the form of a servant, but He was free from the sins and imperfections of the human body: that we might be in Him, because He was born of the Virgin, and yet our faults might not be in Him, because He is the source of His own humanity, born as man but not born under the defects of human conception.
It is this mystery of His birth which the Apostle upholds and demonstrates, when he says, He humbled Himself, taking the form of a servant, being made in the likeness of a man and being formed in fashion as a man: that is, in that He took the form of a servant, He was born in the form of a man: in that He was made in the likeness of a man, and formed in fashion as a man, the appearance and reality of His body testified His humanity, yet, though He was formed in fashion as a man, He knew not what sin was.
For His conception was in the likeness of our nature, not in the possession of our faults. For lest the words, He took the form of a servant, might be understood of a natural birth, the Apostle adds, made in the likeness of a man, and formed in fashion as a man. The truth of His birth is thus prevented from suggesting the defects incident to our weak natures, since the form of a servant implies the reality of His birth, and found in fashion as a man, the likeness of our nature.
He was of Himself born man through the Virgin, and found in the likeness of our degenerate body of sin: as the Apostle testifies in his letter to the Romans, For what the law could not do, in that it was weak through the flesh, God sending His Son in the likeness of flesh of sin, condemned sin of sin. He was not found in the fashion of a man: but found in fashion as a man: nor was His flesh the flesh of sin, but the likeness of the flesh of sin. Thus the fashion of flesh implies the truth of His birth, and the likeness of the flesh of sin removes Him from the imperfections of human weakness.
So the Man Jesus Christ as man was truly born, as Christ had no sin in His nature: for, on His human side, He was born, and could not but be a man; on His divine side, He could never cease to be Christ. Since then Jesus Christ was man, He submitted as man to a human birth: yet as Christ He was free from the infirmity of our degenerate race.
Source: On the Trinity (New Advent)