Ontological argument
There are different versions, but they all argue something like: because we can imagine a perfect being, there must be a god. The idea is that existing makes a good thing better than one that's only imaginary. So the perfect thing we're imagining must exist. Then we call the perfect thing God. The earliest objection was that an argument like that could prove wrong things. You could prove that a perfect island must exist, for example. But no real island is perfect.