Let G(A) be the probability a person A assigns to the existence of (a personal) God. We have that (i) if 0 <= G(A) < 0.5, then A is an atheist; (ii) if G(A) = 0.5, then A is an agnostic; and (iii) if 0.5 < G(A) <= 1, then A is a theist.
Let us assume you are A, what is the value of G(A)?
P.S. Note that as, strictly speaking, one cannot be absolutely certain of anything, G(A) = 0 or G(A) = 1, are clearly unreasonable positions.