So...what happened?

The Pope's visit to the UK happened to coincide with Yom Kippur, the Jewish day of atonement. While that was entirely coincidental, it did bring into sharp relief the differences between these two religions, especially considering one was ostensibly an outspring of the other.  So what happened?

First, a pretty strong disclaimer: I'm not a scholar, I'm not an expert, I'm not even very religious.  And most of my information comes from Wikipedia, because what better source for contentious, heretical, and blasphemous information than a publicaly editable database?

Throughout its early years, Israel was largely a subject of other empires: Babylonia, Persia, Macedonia (Greece), Seleucid, and Rome.  During a brief period (the Maccabean Revolt, 164 BCE to 63 BCE), Israel became independent.  Unfortunately, John Hyrcanus, third son of Simon Maccabaeus, took the throne after his father and two brothers were killed at a banquet under suspicious circumstances.  He then went on a series of conquests, capturing Trans-Jordan, Samaria, Galilee, and Idumea, and forcing them to convert to Judaism.  That was the first time anyone had been forced to convert to Judaism, and it had significant consquences.

Despite a rather unlikely story putting him in Bethlehem (and born of a virgin mother), Jesus was probably born in Galilee, one of the conquered territories.  He was probably Jewish in name only.  He had a wholly unremarkable childhood, then at 30 he went to the river Jordan where John the Baptist was performing mikvahs, a Jewish bathing ceremony, and he began 'ministering' himself. It's not entirely clear what he was ministering, but it didn't matter much -- a year later, he was arrested and crucified by the Romans.

He did establish Christianity as a separate Jewish sect, but it was one of many: Pharisees, Sadducees, Zealots, and Essenes.  To convert to Christianity, gentiles first had to convert to Judaism -- i.e. become circumcised, keep kosher, observe the Sabbath, etc. 

The sect would have likely died out, since it believed that Jesus was the messiah who would return to free Jerusalem from the Romans, and that was never going to happen.  However, 14 years after the death of Christ, and for reasons unknown, Saul of Tarsus, a Pharisee who had never met Jesus, began trying to convert Jews to this new sect.  Rejected, he then turned to gentiles (non-Jews) but claimed they did not have to convert to Judaism to accept Jesus.  In 50 CE, Judaism and Christianity split entirely; the Mosaic law was rejected, and Paul created his new church, with Jesus as the spiritual messiah.

Why so many pagans chose to embrace Christianity, and how the Catholic church came to dominate Christianity, are stories for another day.

Interesting sidenote: After the Romans destroyed the second temple in 70 CE, and later expelled Jews from Jerusalem, Judaism changed -- the Sadducees, Zealots, and Essenes faded into history.  Only the Pharisees survived the diaspora, adapting what is now known as 'Rabbinic Judaism.'