The Enchiridion 37
37 The Same Grace is Further Clearly Manifested in This, that the Birth of Christ According to the Flesh is of the Holy Ghost PL 40, 251
For the same Jesus Christ who is the only-begotten, that is, the only Son of God, our Lord, was born of the Holy Ghost and of the Virgin Mary. And we know that the Holy Spirit is the gift of God, the gift being Himself indeed equal to the Giver. And therefore the Holy Spirit also is God, not inferior to the Father and the Son. The fact, therefore, that the nativity of Christ in His human nature was by the Holy Spirit, is another clear manifestation of grace. For when the Virgin asked the angel how this which he had announced should be, seeing she knew not a man, the angel answered, “The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.” And when Joseph was minded to put her away, suspecting her of adultery, as he knew she was not with child by himself, he was told by the angel, “Fear not to take unto thee Mary thy wife; for that which is conceived in her is of the Holy Ghost:” that is, what thou suspectest to be begotten of another man is of the Holy Ghost.
Nativitas Christi eo quod est de Spiritu sancto, gratiam demonstrat. Idem namque Jesus Christus Filius Dei unigenitus, id est unicus, Dominus noster, natus est de Spiritu sancto et virgine Maria. Et utique Spiritus sanctus Dei donum est, quod quidem et ipsum est aequale donanti: et ideo Deus est etiam Spiritus sanctus, Patre Filioque non minor. Ex hoc ergo quod de Spiritu sancto est secundum hominem nativitas Christi, quid aliud quam ipsa gratia demonstratur? Cum enim virgo quaesivisset ab angelo, quomodo id fieret quod ei nuntiabat, quandoquidem illa virum non cognosceret; respondit Angelus: Spiritus sanctus superveniet in te, et virtus Altissimi obumbrabit tibi; ideoque et quod nascetur ex te sanctum, vocabitur Filius Dei . Et Joseph cum vellet eam dimittere, suspicatus adulteram, quam sciebat non de se gravidam, tale responsum ab angelo accepit: Noli timere accipere Mariam conjugem tuam; quod enim in ea natum est, de Spiritu sancto est : id est, quod tu esse de alio viro suspicaris, de Spiritu sancto est.