Different Jews will give different opinions on what the main principle of Judaism is.
Here are some principles that usually get labeled "the main idea":
The spelling of the doctrine within Judaism is Kaballismbut often just Kaballah.
Reformed Judaism believes that the Bible is the main source for the rules of Judaism.
Doctrine in the early Christian church helped unify the people by helping them to understand the differences between Judaism and the new form of Christianity - Roman Catholic.
Moses.
Israel.
The Torah.
JUdaism
Jesus' followers believes/ed that he is the Messiah, something that Judaism does not believe.
A proselyte can refer to a new convert to a religious faith or doctrine. In historical contexts, a proselyte can also refer to a Gentile who converted to Judaism.
Judaism does not have a centralized leadership.
Abraham and Moses.
The main religion in Israel is Judaism with 76.4% of the population following it.