But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, son of David, fear not to take unto you Mary your wife: for that which is conceived in her is of the Holy Spirit.
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The very thing which had made him afraid and for which he would have cast her out— this very thing, I say, was a just cause why he should take her and retain her in his house. This more than entirely did away with his distress. It is as if the angel were saying, “For she is not only free from unlawful sexual relations but her very conception is above all natural causes. So not only put away your fear but rejoice even more greatly, ‘for that which is conceived in her is of the Holy Spirit.’” What a strange thing he spoke of, surpassing human reasoning and all the laws of nature. How then might one be made able to believe such an announcement that would be so wholly unexpected? Only by viewing this event in relation to past disclosures in Scripture. For with this intent the angel laid open to Joseph all things that were in his mind, what he felt, what he feared, what he was resolved to do, so that he would be wholly reassured. And not only by past revelations in Scripture but also by the promise of what is yet to come does the angel win him over: “She will bear a son, and you shall call his name Jesus.” The Gospel of Matthew, Homily