What religion is not only practiced today but also served as the foundation for both christianity and islam?

Judaism.

Explanation:

Judaism was the foundation for both Cristianity and Islam, from the practices and old texts of judaism both religions are born, Christianity with the idea that Jesus was the savior that was promised, and islam seeing him as a prophet, that came as many others to spread the word of God while living on the Earth.

answer; underachivement;