So then we confess that God fulfilled the promise which He had made to the early fathers by the mouth of His holy prophets when He sent His only and eternal Son into the world at the time set by Him.
The Son took the "form of a servant" and was made in the likeness of man, truly assuming a real human nature, with all its weaknesses, except for sin; being conceived in the womb of the blessed virgin Mary by the power of the Holy Spirit, without male participation.
And He not only assumed human nature as far as the body is concerned but also a real human soul, in order that He might be a real human being. For since the soul had been lost as well as the body He had to assume them both to save them both together.
Article 18: The Incarnation