That Christ was conceived by the Holy Ghost

THOUGH every divine activity, whereby anything is done in creatures, is common to the entire Trinity, nevertheless the formation of the body of Christ is appropriately attributed to the Holy Ghost: for in Scripture every grace is wont to be attributed to the Holy Ghost, since what is given gratuitously is reckoned to be bestowed out of the love of the giver;* and there is no greater grace bestowed on man than his coming to be united with God in union of person.* Still the Holy Ghost cannot be called the father of Christ in His human generation; because the Holy Ghost did not produce the human nature of Christ out of His own substance, but merely by an exertion of His power.

4.45 : That Christ was born of a Virgin without prejudice to His true and natural Humanity
4.54 : Of the Incarnation as part of the Fitness of Things