No, that only became popular after the Romans started doing it. Before then, most religions were polytheistic. And that word has two definitions. It doesn't just mean believing in multiple gods, it's also used to refer to believing in all gods. The ancient greeks didn't have any problem accepting the existence of Ra, or Ishtar, or the gods of other nearby countries. They just didn't worship those gods, because Ra is off doing Egyptian stuff in Egypt and Ishtar is busy in Babylon. They worshipped Greek gods because the Greek gods were the ones who actually hung out with the Greeks. And this is true long before the Greeks even began to think of themselves as Greek.

A big reason why the Romans were able to conquer such a massive empire is religious genocide. See, most empires would conquer a country, and then the locals would spend dozens or hundreds of years being angry at the oppressors because they spoke a different language, worshipped different gods, and practiced different rituals. The Romans invented the practice of "Hey buddy, we don't actually follow different religions, we just have different interpretations. Your chief god is actually just a different interpretation of Jupiter, and your war god is just another name for Mars. We'll start telling your myths and doing your rituals from now on as long as you start speaking Latin, okay buddy?" They destroyed the cultural distinctiveness of their subjects to pacify them, claiming there was only one pantheon. Eventually the Romans adopted Christianity, claimed there was only one god, and the world you know today was created.