16 Not to touch upon the vain enquiries of other heretics— concerning whom however, when the course of my argument gives occasion, I will not be silent— there are those who tamper with the faith of the Gospel by denying, under the cloak of loyalty to the One God, the birth of God the Only-begotten. They assert that there was an extension of God into man, not a descent; that He, Who for the season that He took our flesh was Son of Man, had not been previously, nor was then, Son of God; that there was no Divine birth in His case, but an identity of Begetter and Begotten; and (to maintain what they consider a perfect loyalty to the unity of God) that there was an unbroken continuity in the Incarnation, the Father extending Himself into the Virgin, and Himself being born as His own Son.
Others, on the contrary (heretics, because there is no salvation apart from Christ, Who in the beginning was God the Word with God), deny that He was born and declare that He was merely created. Birth, they hold, would confess Him to be true God, while creation proves His Godhead unreal; and though this explanation be a fraud against the faith in the unity of God, regarded as an accurate definition, yet they think it may pass muster as figurative language. They degrade, in name and in belief, His true birth to the level of a creation, to cut Him off from the Divine unity, that, as a creature called into being, He may not claim the fullness of the Godhead, which is not His by a true birth.
Source: On the Trinity (New Advent)