Amongst Jews, the most important verse of the Torah is the Shema - Deuteronomy 6:4:
Hear, O Israel: The LORD our God, the LORD is one.
That is the distinctive call of the Hebrew religion, the first major monotheistic impulse. It is specific revelation, as opposed to general revelation, so one would not expect a non-biblical answer. One does not get specific revelation from observation or logic - it requires faith by definition.
It dates to the time of Moses, and is a radical departure from all that surrounded it.
Fast forward more than a milennium, and it is to these Jews that Jesus first came. Indeed, initially, Jews and Christians were hard to distinguish!. Thus, the same Jesus who said "I have not come to abolish the Law but to fulfill it" would necessarily have been telling his followers to be monotheistic.
Muslims like to claim that Christians reject monotheism via the Trinity, but that is an incorrect understanding of Christainity. (As an aside, one my way to church on Sunday, I saw a license plate that said "1 not 3". I knew the driver had to be Muslim, and sure enough, as she passed me, she was wearing an hijab. Her understanding was simply wrong.)