50 Further their heretical ingenuity presses on in the path prepared by their own godlessness, even to the entire absorption of God the Word into the human soul, and consequent denial that Jesus Christ, the Son of Man, was the same as the Son of God. So either God the Word ceased to be Himself while He performed the function of a soul in giving life to a body, or the man who was born was not the Christ at all, but the Word dwelt in him, as the Spirit dwelt in the prophets. These absurd and perverse errors have grown in boldness and godlessness till they assert that Jesus Christ was not Christ until He was born of Mary. He Who was born was not a pre-existent Being, but began at that moment to exist.
Hence follows also the error that God the Word, as it were some part of the Divine power extending itself in unbroken continuation, dwelt within that man who received from Mary the beginning of his being, and endowed him with the power of Divine working: though that man lived and moved by the nature of his own soul.
Source: On the Trinity (New Advent)