There are two ways of defining Monotheism: A) Our people worship a single god and he is greater than all other gods; and B) There is only one god, and all claims to the contary are simply false.
There is significant evidence that the Hebrews belief evolved slowly from A to B over the course of centuries. By the time Christianity was born, B was pretty well established.
Zoroastrianism presents Ahura Mazda as the "Supreme God" and predates Christianity. But is that "supreme" as in, the most powerful god? Or supreme as in the only god? I don't know enough about it to say one way or another. Zoroastrianism is still practiced.
D&D DMs have the advantage of being able to lay out the ultimate truth in black and white, a luxury that we lack on the real world. "Of course such-and-such is a real god! See, he grants me spells!" as opposed to "Of course such-and-such is a real god! Well, no, he doesn't grant me spells..." Hard to argue with, really. When the priests of a god can do magic, it's pretty good proof that their god exists. If only the worshippers of one god can do magic, then it's a monotheistic world, regardless of how many false religions there are.