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. ”

“ For the greater the power of an agent, the more quickly can it dispose matter; and, consequently, an agent of infinite power can dispose matter instantaneously to its due form. Secondly, on the part of the Person of the Son, whose body was being formed. For it was unbecoming that He should take to Himself a body as yet unformed. While, if the conception had been going on for any time before the perfect formation of the body, the whole conception could not be attributed to the Son of God, since it is not attributed to Him except by reason of the assumption of that body. ”