That's your proposition of truth regarding the nature and existence of God/gods. And presumably it's based on some kind of reasoning. In a philosophical setting such as this, when you posit a proposition of truth, you are expected to offer the reasoning by which you arrived at it, for an analysis of the logic within it, by those you proposed your truth, to. Just as when the theist proposes theism as a truth claim, to you, he is likewise expected to offer the reasoning by which he arrived at that position, for logical analysis, by you, and others.
You can claim your "disbelief" as your justification for asserting this truth claim, but what you believe or don't believe carries no weight at all, logically. And so will rightly be ignored by anyone seeking a logical reason to accept your assertion as valid.