Mankind was one nation having one religion. Later, when people invented other religions, Allah appointed Prophets as bearers of good news and warnings; and revealed to them the Book with the True Guidance to settle the matters of dispute between people. 2: 21

O people of the Book (Christians and Jews) let us get together on what is common between us and you: that we shall worship none but Allah; that we shall not associate any partners with Him; that we shall not take from among ourselves any lords beside Allah. 3: 64

Say: "We believe in Allâh and that which has been sent down to us and that which has been sent down to Abraham, Ishmael, Isaac, Jacob, and to Al-Asbât [the twelve sons of Jacob], and that which has been given to Moses and Jesus, and that which has been given to the Prophets from their Lord. We make no distinction between any of them, and to Him we have submitted."

So if they believe as ye believe, they are indeed on the right path; but if they turn back, it is they who are in schism; but Allah will suffice thee as against them, and He is the All-Hearing, the All-Knowing.

Quran: Al-baqarah, verse 136 & 137