Chapter 5 — Part I: The Creed — Article III: Who Was Conceived by the Holy Ghost, Born of the Virgin Mary
Section 9. But what surpasses the order of nature and human comprehension is, that as soon as the Blessed Virgin assented to the announcement of the Angel in these words, Behold the handmaid of the Lord; be it done unto me according to thy word, the most sacred body of Christ was immediately formed, and to it was united a rational soul enjoying the use of reason; and thus in the same instant of time He was perfect God and perfect man. That this was the astonishing and admirable work of the Holy Ghost cannot be doubted; for according to the order of nature the rational soul is united to the body only after a certain lapse of time.