There is a strong Christian tradition of seeing faith as involving certainty. Now, perhaps this certainty is just something like moral certainty (for a sophisticated account, see this) or what I call "security" or "sureness". But it is worth interesting to explore the possibility that faith involves certainty in the full sense of the word, requiring a probability equal to one to be assigned to those propositions that come under faith (this does not imply that faith is exhausted by believing propositions).
There are at least two problems with a certainty reading of faith. The first is with justification: How could someone be justified in assigning a probability of one to propositions as controversial as those of the Christian faith? The second starts with an empirical claim: Typical Christians do not have certainty. Given this, it follows that if faith requires certainty, then typical Christians do not have faith, which is definitely depressing and perhaps not so plausible.
But I think there is a way around both difficulties. The Christian tradition sees faith as a gift of the Holy Spirit. It does not seem problematic that the Holy Spirit would infuse someone with a certainty about a truth that the Holy Spirit himself knows with certainty. There are at least two possibilities here. First, it could be that the right kind of externalism (e.g., reliabilism or reformed epistemology) holds so that the certainty of the beliefs that come from such an infusion is epistemically justified. Second, it could be that there is nothing bad about having epistemically unjustified beliefs when they are in fact true and when the agent is not at fault for their formation and maintenance. It would be better to have justification as well, but it's better to have the true beliefs than to suspend judgment. And, plausibly, the story of the infusion of belief could be spelled out in a way that does not make the agent be at fault.
Regarding the empirical problem, here I am less confident. But here is a suggestion. What makes us think that typical Christians do not have certainty? Presumably that they so report it. Or so I'll assume for the sake of the argument, without spending time checking if there are any worldwide studies that check whether typical Christians report not having certainty.
Presumably, introspection is the primary reason why Christians report not having certainty. But I suspect that introspection is not a very reliable guide in a case like this. It seems to me that there are two primary ways by which we introspect the credence we have in a belief. The first is that we introspect to the evidence we take ourselves to have and assume that our credence matches the evidence. But when our certainty goes beyond the evidence, or at least goes beyond the evidence that we are aware of, this isn't going to be a reliable guide to the credence. The second is direct awareness (often comparative) of our credences. Often this is based on feelings of confidence. But such feelings are, I think, not all that reliable. They provide evidence as to actual confidence, but that evidence is not all that strong. While we should try to avoid error theories all other things being equal, it does not seem so bad to say that Christians tend to be wrong when they ascribe to themselves a credence lower than one.
I am inclined to take the best reading of the Christian tradition to be that faith comes with certainty. I also think that I have faith, but I do not in fact feel certain (at least not in the probability one sense). Given that my best reading of the Christian tradition is that faith comes with certainty, I conclude that probably I am certain, notwithstanding my feelings.