Late as usual, but as a personal fan of deities in fantasy, I find this an interesting conundrum. In some cases, deities existed before their worshipers--indeed, many created their worshipers. In FR, I thought the need for worship to gain power was a post-ToT thing, but someone else here pointed out that isn't the case. Maybe upon creating a race/species, they then essentially became beholden to it. But if that were the case, it may make other gods reluctant to follow suit.
For those who follow Critical Role, possible spoilers below
Recently in CR, while venturing into weird city of Aeor, Calab found a journal in which the writer commented on the debate that the gods were formed by mortals. For one thing, this was a city that defied the gods, wanting to bring them down--and it didn't end well for them, so this could be pure speculation on their part, as they had a negative view of deities, anyway. But, regardless, I think the word formed is key here. While often synonymous with create, I don't think this is the case here. In C1, during the VM's talk with the goddess Ioun, she mentioned that her realm was her essence, and that the avatar they were conversing with was people's faith given form. Likewise, the Explorer's Guide to Wildemont said that in the beginning, the gods were formless, and were such when they created the elves, dwarves, and humans. So, mortals didn't create the gods, but their faith allowed the gods to take form--ie, have things like an avatar. On a practical level, this makes sense in that the mortal mind probably couldn't comprehend the "true" form of a deity.
So, if we go by this logic, then gods are dependent on mortals to give them form. Personally, this sits better with me than the idea of mortals creating the gods, especially in the case when deities (and having them) help determine the afterlife. The trope of gods being created solely from human faith seems like an attempt to give mortals more power and agency, which on the surface may sound ideal, but having gods doesn't mean mortals are deprived of agency and free will. This idea also just seems like a snub toward gods in general, as if anything having to do with divinity is bad, even in fiction. I'm not very religious, but divinity in fantasy can add to a setting, and mortals creating gods begs the questions: who/what created the mortals, and what about the afterlife? How would that work? Does everyone go to the same place, like in some real world beliefs? In Planescape, even the faithless have a chance at an afterlife, unless they don't even believe they possess a soul (at least from my understanding. My only real experience with Planescape is On Hallowed Ground, but it seems like even the faithless get a chance to be with a god, unless they dont' even believe in themselves). This may not matter much in the average campaign, but in cosmological discussions, I think it's worth considering.
This actually makes me think of the opposite of "seeing is believing". Think of stories where certain creatures are only visible to children, because children have their "eyes wide open", and don't yet have the skepticism and filters of adults. Instead of seeing is believing, you have to believe to see. So in a system that is shaped by belief, perhaps it is that we have to believe in order to have communication of any kind with deity. It's not that they can't exist without faith, but the mortal's faith sustains their ties to the mortals, and allows them to interact. It keeps them anchored. Without the connection of faith, the ties between deity and mortal are severed.
