There's actually another relevant MTG story to your question (though it still stops short of exactly answering it)... It's called Dreams of the City.

TL;DR: A villainous Planeswalker decides to study what ascension to godhood means on Theros, and he implants visions of a potential god in the nightmares of thousands of people.

He succeeds, and a new God, Cacophony is birthed. As it goes:

Xenagos had proven turning a mortal into a god was difficult, but possible. But turning an idea into a god was much easier. And for those who had the ability to control dreams, it was simpler still. Ashiok suspected ideas took shape and became proto-deities in Nyx all the time, but were probably subsumed into the existing deities without anyone, including the gods, aware it was happening.

Belief alone isn't enough to stick around as a god on Theros, though... Ephara discovers baby god quickly, and murders it.