Matthew 1:18

The account of the birth of Jesus Christ follows his genealogy. From these words, "before they came together," Helvidius and others have started objections, which have been answered long ago by St. Jerome, where he shews in many examples from Scripture, that the words before and until do not signify what happened afterwards; for that point is left indefinite, but only what was done before, or not done. Thus when it is said, Sit thou at my right hand, till I make thine enemies they footstool, Psalm cix, by no means signifies, that after the subjection of his enemies, the Son of God is no longer to sit at the right hand of his Father. In common conversation, when we say that a man died before he reached his 30th year, we do not mean that he afterwards attained it. Or, should we say that Helvidius died before he did penance, we cannot mean that he afterwards did penance: the same conclusion should be deduced from the words, "before they came together," the end being accomplished by the power of the operation of the Holy Ghost, without their going together. If we should advance, that such a man was cured before he went to a physician, the natural inference would be, that he did not go to a physician at all. Thus also in the language of Scripture, the word first-begotten does not mean after whom others were born, but before whom no one was born, where there were further issue or not. And the reason is, because the law required that a sacrifice should be offered for the first-born, and that he should be redeemed very soon after his birth; nor did it allow the parents to wait and see if any other son should be born. (Estius) --- True and perfect marriage, and continual living in the same, without knowing each other. (St. Augustine, lib. ii. Consen. Evang. chap. i.) (Bristow)