I think you're misunderstanding something very fundamental. Christianity was founded at least a thousand years after Judaism in a recognizable form was founded. The most noticeable difference between Judaism and Christianity is Christ. Christians believe that Jesus Christ was the Messiah, and died on the cross to atone for the sins of man. Jews believe that the Messiah has not yet come, and do not believe in the divinity of Jesus. I can't speak much for what exactly happened to Christianity that it exists today in its varied forms (including Catholicism), but Judaism's most defining moment was the destruction of the Temple in Jerusalem in 70 CE. The Temple was the focal point for Jewish life, and the subsequent exile from the land of Israel threatened the entirety of Jewish identity. A few centuries later, Jewish law was formally codified in the Talmud, encompassing what had previously been an oral tradition and elaborating on that with the teachings of Jewish sages. Over the next millennium and a half, as Jews spread throughout the world, they brought the traditions with them, and differences in practice and custom arose, leading to today's diversity of Jewish identity and practice.

Here's a concise timeline of Jewish history (there are some Christian and Muslim events as well, but other than the more well-known ones, I can't confirm them off the top of my head). Jewish Virtual Library timeline.