It's interesting that in religion back then they took the idea of the gods' existence as an obvious fact. So when they encountered other people with different gods they assumed that they were actually the same gods, just by different names. If they encountered civilized/intelligent people with different mythologies it meant either that those people knew something the Greeks previously didn't or they had just gotten events confused and it would become a matter of debate. It was more like two people with different, but honest, accounts of an event in the remote past trying to reconcile their own limited recollection or view point.
I guess for the ones associating their gods with real phenomena, it seems almost inevitable.
> So when they encountered other people with different gods they assumed that they were actually the same gods, just by different names. [...] It was more like two people with different, but honest, accounts of an event in the remote past trying to reconcile their own limited recollection or view point.
And not just with the classic Greek/Roman Gods. The similarities between Pandora and Eve, the Flood myths[0] like Deucalion and Noah; there are so many appropriated stories in the Abrahamic religions (probably others, but most familiar with Greek and Abrahamic ones) I don't even understand how these days religion is taken seriously given the historical pattern of appropriation and cleansing of the more fantastical shape-shifting, creature, magic-spell type bits.
[0] https://en.wikipedia.org/wiki/Flood_myth