On the belief of the “Chosen People” in the creed of Judaism

In many internet threads, i’ve read the term “The Chosen People” referring to the Jewish people. This is supposedly a belief shared by all practicing Jews because it is explicitly expressed in the teachings and texts of their religion.

Now, i’m writing this post to clarify the doubt i have about this matter and the supposed belief in “the Chosen People” among Jews. Is it true that this belief is part of the creed, theory, and teachings of the Jewish religion?

If this is true, what is the explicit source that supports this shared belief among Jews?

Finally, i have another question, not about the beliefs and creed of Judaism, but about Christianity in relation to Judaism:

If the belief in being the “Chosen People” is part of the creed of Judaism, is this belief also part of the creed of Christianity?