Luke 1:36

(Carm. 18. de Geneal. Christi.) But some one will ask, How is Christ related to David, since Mary sprang from the blood of Aaron, the angel having declared Elisabeth to be her kinswoman? But this was brought about by the Divine counsel, to the end that the royal race might be united to the priestly stock; that Christ, Who is both King and Priest, might be descended from both according to the flesh. For it is written, that Aaron, the first High Priest according to the law, took from the tribe of Judah for his wife Elisabeth, the daughter of Aminadab. (Exod. 6:23.) And observe the most holy administration of the Spirit, in ordering that the wife of Zacharias should be called Elisabeth, so bringing us back to that Elisabeth whom Aaron married.

(49 in Gen.) Seeing that his previous words had overcome the mind of the virgin, the angel drops his discourse to a humbler subject, persuading her by reference to sensible things. Hence he says, And, behold, Elisabeth thy cousin, &c. Mark the discretion of Gabriel; he did not remind her of Sarah, or Rebecca, or Rachel, because they were examples of ancient times, but he brings forward a recent event, that he might the more forcibly strike her mind. For this reason also he noticed the age, saying, She also hath conceived a son in her old age; and the natural infirmity also. As it follows, And this is the sixth month with her who was called barren. For not immediately at the beginning of Elisabeth's conception did he make this announcement, but after the space of six months, that the swelling of her womb might confirm its truth.

So it was then, lest the virgin should despair of being able to bear a son, that she received the example of one both old and barren about to bring forth, in order that she might learn that all things are possible with God, even those which seem to be opposed to the order of nature. Whence it follows, For there shall be no word (verbum) impossible with God.