17 The blessed Apostle also perfectly describes this mystery of the ineffable birth of Christ's body in the words, The first man was from the soil of the ground, the second man from heaven. Calling Him 'Man' he expresses His birth from the Virgin, who in the exercise of her office as mother, performed the duties of her sex in the conception and birth of man. And when he says, The second man from heaven he testifies His origin from the Holy Ghost, Who came upon the Virgin. As He is then man, and from heaven, this Man was born of the Virgin, and conceived of the Holy Ghost. So speaks the Apostle.
Source: On the Trinity (New Advent)