To me, a god is something thought to be personable (i.e. it has associated stories and is humanlike) and supernatural (i.e. it has special powers). We could tell that one god is different from another god if they had different stories, personalities, and powers. Clearly different religions have different gods with different stories, personalities, and powers, and many religions even have multiple gods. Hence there hasn't been only one God throughout all the religions, there have been many.
I'm an atheist, but I think we can look at this through a purely anthropological lens, so my beliefs shouldn't matter too much in this conversation. Certainly many cultures, both modern and historical, would tell you that there are many gods, and plenty of other cultures would tell you that their god is different from all other gods. I'm curious about how you would reconcile this.
It's pretty easy to find religions that don't match that pattern, though. Buddhism doesn't say that your "soul goes up to God." Germanic paganism didn't think that everyone would meet one particular god in the afterlife. Etc.