While there are several different interpretations of probability, even more than one objective interpretation, it is hard to see how to assign anything other than a subjective probability to the existence of an entity so ill-defined as a god.
Of course the subjective probability is whatever you want it to be, so you could say that it is 0 or 1 if you like. Pascal in the famous wager seems to assign probability .5 to the existence of God—there are just two (equally likely) possibilities. There does not seem to be any justification for this.
In any case assigning any probability to the existence of a god is really questionable.
In my opinion, saying you are 100% sure is not a lot better (in intellectual terms) than being a true believer. The point is no one can say for certain, but after examining all the evidence (or complete lack thereof), I feel very confident! I always explain it the way Richard Dawkins does: no one can say for certain that fairies don't exist either, but given the lack of evidence, it is absurd to believe they do. Belief in a god is the same.
Subjectively I'm 100% certain there is no god. Objectively I have to allow for my own bias , therefore a god has to be a possibility outside of my ignorance. But the question being am I sure there's no god? Yes I'm sure there's no god.