The New Testament begins with a genealogy of Jesus. Genealogies matter greatly to the Jews as they establish legitimacy and inheritance which is especially important since part of the claim is that Jesus is the heir of King David. Mary was betrothed to Joseph but already pregnant with Jesus.

Joseph was going to divorce her but an angel (identified later as Gabriel) appeared to explain the situation: namely that his wife-to-be had been impregnated BY God WITH God, Jesus Christ, Immanuel, God Among Us (don’t do it), the Anointed One promised to Abraham to bless the whole world. So I figure that would be a lot to take in. Being a faithful man, Joseph rolls with it. There’s trusting God and there’s really really trusting God and Joseph’s willingness to accept this should speak volumes of his character. He could’ve gone the other direction and just accused Mary of infidelity with a Roman soldier, as the Jewish Babylonian Talmud does in effort to explain away Christ. Looking into it, they refer to Christ as “Yeshu” instead of “Yeshua” and call His mom unfaithful as deliberate mockery in response to the rise of the early church. Unlike other heresies, you don’t see much of this one, so it may have just been some early rabbinical seething and counter propaganda that just didn’t stick.

Mary’s virginity isn’t just of concern to them, of course. Here it’s stated that Joseph didn’t “know” Mary until Jesus was born. The Protestant read of this follows normal English which is “okay then he did after” whereas Catholic says “He didn’t do it before and he didn’t do it after” because the language originally used makes no claim to “after” just because “before” is mentioned. So both sides have a valid case to claim. We’ll get into “Jesus’s brothers” later.