51 Through this subtle and mischievous doctrine they are drawn into the error that God the Word became soul to the body, His nature by self-humiliation working the change upon itself, and thus the Word ceased to be God; or else, that the Man Jesus, in the poverty and remoteness from God of His nature, was animated only by the life and motion of His own human soul, wherein the Word of God, that is, as it were, the might of His uttered voice, resided. Thus the way is opened for all manner of irreverent theorising: the sum of which is, either that God the Word was merged in the soul and ceased to be God: or that Christ had no existence before His birth from Mary, since Jesus Christ, a mere man of ordinary body and soul, began to exist only at His human birth and was raised to the level of the Power, which worked within Him, by the extraneous force of the Divine Word extending itself into Him.
Then when God the Word, after this extension, was withdrawn, He cried, My God, My God, why have You forsaken Me? or at least when the divine nature of the Word once more gave place within Him to a human soul, He Who had hitherto relied on His Father's help, now separated from it, and abandoned to death, bemoaned His solitude and chid His deserter. Thus in every way arises a deadly danger of error in belief, whether it be thought that the cry of complaint denotes a weakness of nature in God the Word, or that God the Word was not pre-existent because the birth of Jesus Christ from Mary was the beginning of His being.
Source: On the Trinity (New Advent)