All this took place to fulfill what the Lord had spoken by the prophet:
22. Now all this was done It is ignorant and childish trifling to argue, that the name Jesus is given to the Son of God, because he is called Immanuel For Matthew does not confine this assertion to the single fact of the name, but includes whatever is heavenly and divine in the conception of Christ; and that is the reason why he employs the...
The mystery of Christ's incarnation is to be adored, not pried into. If we know not the way of the Spirit in the formation of common persons, nor how the bones are formed in the womb of any one that is with child (Ecc 11:5), much less do we know how the blessed Jesus was formed in the womb of the blessed virgin.
Commenting on Matthew 1:18-25
Now all this was done,.... These are not the words of the Angel, but of the Evangelist; observing that Mary's being with child of the Holy Ghost, and her conception in such an extraordinary manner, whilst a pure virgin, before she and Joseph came together, who though espoused to him, was untouched by him, were all brought about in this way, and with such circumstances...