Judaism is a perversion of the Hebrew religion that primarily came as a result of a century under Babylonian rule (Which is also how the Jews got their "Bible", the Talmund).

What is the Mishnah?
Compiled around 200 by Judah the Prince, the Mishnah, meaning 'repetition', is the earliest authoritative body of Jewish oral law. It records the views of rabbinic sages known as the Tannaim (from the Aramaic 'tena', meaning to teach).
The Torah – the first five books of the Hebrew and Christian Bibles, as handed down by God to Moses – forms the basis of Jewish written law. The Mishnah supplements the Torah, but its laws lack all scriptural references.
The Mishnah was the first great creation of the rabbis, who were not clergy but rather came from all walks of life. It is a large collection of sayings, arguments and counter-arguments that touch on virtually all areas of life (including Temple ritual, which was long gone).
What is the Talmud?
Over the three centuries following the compilation of the Mishnah, the rabbis added enormously to it with the Gemara: what they called a 'sea' of learning, using stories about both Biblical personalities and their own lives, sober legal arguments and fanciful imaginings of the world of old and the world to come. Around 500 AD they wrote it all down in multiple volumes that go under the name Talmud, 'Teaching'.
...
Moses Maimonides (1135-1204) was a Jewish rabbi, physician and philosopher in Spain, Morocco and Egypt. His religious writings were highly influential. In his Mishneh Torah, completed in 1180, he organised, edited, summarised and codified the laws in the Talmud. This is probably is the greatest individual contribution to Jewish law. He also wrote a Commentary on the Mishnah, one of the first works of its kind. Its contents are widely quoted and appear on the page illustrated above.

Oral teachings being recorded as written text doesn't mean a shift in core beliefs or practices; it's not creating a 'new' religion.