"god exists" <Why is this a relevant argument?

[Tiberius]

Propositions are statements that are either true or false.

For instance, the statement "Tiberius is a man" is a proposition that happens to be true, and I can provide arguments that can assert that truth.

Other propositions are "Snow is white", "All roads are straight", "Homosexuality is natural", etc.

So yes, "God exists" is a proposition. "God doesn't exist" is another proposition. Such statements can be true or false, and there are varying arguments for both.