Thomas Aquinas

“ Art. 2 - Whether Christ's body was animated in the first instant of its conception? Objection 1: It would seem that Christ's body was not animated in the first instant of its conception. For Pope Leo says (Ep. ad Julian.) : "Christ's flesh was not of another nature than ours: nor was the beginning of His animation different from that of other men." But the soul is not infused into other men at the first instant of their conception. Therefore neither should Christ's soul have been infused into His body in the first instant of its conception. ”

“ What the Philosopher says is true in the generation of other men, because the body is successively formed and disposed for the soul: whence, first, as being imperfectly disposed, it receives an imperfect soul; and afterwards, when it is perfectly disposed, it receives a perfect soul. But Christ's body, on account of the infinite power of the agent, was perfectly disposed instantaneously. Wherefore, at once and in the first instant it received a perfect form, that is, the rational soul. ”

“ I answer that, As stated above, we may say properly that "God was made man," but not that "man was made God": because God took to Himself that which belongs to man---and that which belongs to man did not pre-exist, as subsisting in itself, before being assumed by the Word. But if Christ's flesh had been conceived before being assumed by the Word, it would have had at some time an hypostasis other than that of the Word of God. ”