In Exodus 20:3 it says "You shall have no other gods besides me" and in Exodus 20:5 it says "You shall not bow down to them or serve them". This seems to acknowledge that there are gods other than Hashem, even though Judaism overall doesn't believe that other gods even exist. Does Hashem believe that there are other gods? Are there other gods but we are not supposed to serve them?
Sforno, commenting on "you shall not bow down to them," says that you shouldn't bow to any phenomena that isn't man made. One of the other commandments says you shouldn't serve idols. This means that you can't pray to a physically man made god, and you also can't pray to a non man made "phenomena." Does this mean you can pray to other gods if they were man made? Would a god automatically be a phenomena? Does flora and fauna count as phenomena, could you pray to them? What about non living, but non man made things? Would something like a rock be considered an idol? Do idols have to be man made?
In this commentary, Rashi gives two explanations of "other gods." One is that "other gods" are not true gods but they were made to be gods by the people worshiping them. The other is a god who is like a stranger to their worshipers and doesn't answer them. Are all gods made by their worshipers? Does this mean they are false? Does Hashem answer our cries? If so is it always obvious, like the splitting of the red sea or the miracle of chanukkah, or is it more subtle like the miracles of Purim, or our day to day lives. If some miracles are subtle, should we not immediately discredit other gods?
In this commentary, Or HaChaim says that as soon as a Jew makes a god, Hashem won't be their god. He also says that if you start worshiping another god, you are bound to start worshiping more and more gods. But is it a bad thing to at least believe in other people's gods? Does that have the same effect that Or HaChaim said that serving other gods would? Even if you don't serve them, wouldn't that help you have empathy towards others. Does only believing in the god you serve cause conflict? Can you still be a monotheist if you believe in, but not serve, multiple gods.
In the end, though it is at the core of Judaism to serve one god, it seems as though Hashem acknowledges the existence of other gods. A lot of people view believing in a god as worshiping it, but that doesn't always have to be true. Not acknowledging that the deity that someone prays is legitimate is delegitimizing their beliefs and therefore it delegitimizes them as a believer. Having a monotheism were you serve one god, instead of only believing in one god could be the key to interreligious peace.