And knew her not till she had brought forth her firstborn son: and he called his name JESUS.
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Theophylact of Ochrid
That is, he never came together with her at all. "Until" here does not mean that before the birth he did not know her and afterwards he did, but that he absolutely never knew her. Scripture employs this expression. For example, the raven "returned not unto the ark until the water had dried off from the earth" (Gen 8:7). But neither did it return after the water had dried off. Again, "I am with you until the end of the world" (Gen 28:20). So after the end He will no longer be with the saints? But how can that be? For at that time more than ever will He be with them. So must you understand here "until she brought forth’’ to mean, neither before the birth nor after the birth did he know her. How could he have touched the Holy Virgin having once understood the ineffable birth giving?
The evangelist does not call Him "her firstborn son" in the sense that she later gave birth to a second son, but simply that He was the first and only child that she bore. For Christ is both the "firstborn" by having been born first, and the "only begotten," in that He had no brother.
And here, too, he shows Joseph’s ready obedience to do everything that the angel had told him to do.