Which major world religion was the first to be monotheistic?

The religion of the ancient Israelites was monotheistic. The Bible in Deuteronomy 6:4 states: "Hear, O Israel: Jehovah our God is one Jehovah."

About Islam: A Qurʼānic commentary states: "The People of the Book went wrong: The Jews in breaking their Covenant, and slandering Mary and Jesus . . . and the Christians in raising Jesus the Apostle to equality with God" by means of the Trinity doctrine.-Surah 4:153-176, AYA

-- Islam is monotheistic.

But even before the Israelites and Islam; Abraham, Noah, believed in the same God as the Israelites. I just don't think there was such a Word as 'religion' back then to differentiate between various beliefs. (ie We don't know by what 'name', Abraham's, and Noah's 'religion' was called.)

I would think that anyone in ancient history who worshipped Jehovah, they were monotheistic.