Concerning our Lord's genealogy and concerning the holy Mother of God
The ever-virgin One thus remains even after the birth still virgin, having never at any time up till death consorted with a man. For although it is written, And knew her not till she had brought forth her first-born Son, yet note that he who is first-begotten is first-born even if he is only-begotten. For the word “first-born” means that he was born first but does not at all suggest the birth of others. And the word “till” signifies the limit of the appointed time but does not exclude the time thereafter. For the Lord says, And lo, I am with you always, even unto the end of the world, not meaning thereby that He will be separated from us after the completion of the age. The divine apostle, indeed, says, And so shall we ever be with the Lord, meaning after the general resurrection.
For could it be possible that she, who had borne God and from experience of the subsequent events had come to know the miracle, should receive the embrace of a man. God forbid! It is not the part of a chaste mind to think such thoughts, far less to commit such acts.
But this blessed woman, who was deemed worthy of gifts that are supernatural, suffered those pains, which she escaped at the birth, in the hour of the passion, enduring from motherly sympathy the rending of the bowels, and when she beheld Him, Whom she knew to be God by the manner of His generation, killed as a malefactor, her thoughts pierced her as a sword, and this is the meaning of this verse: Yea, a sword shall pierce through your own soul also. But the joy of the resurrection transforms the pain, proclaiming Him, Who died in the flesh, to be God.
Source: Exposition of the Faith (New Advent)