I once heard that if you're not 100% sure (about Christianity), you're 100% lost. I couldn't disagree more. I would rephrase it like this: If you are 100% sure, you don't have faith.
Faith does not mean "belief without evidence", but it does imply uncertainty. A good definition of faith is "confident trust despite uncertainty". If you're 100% sure because you have absolute knowledge, you have no need for faith. If you're 100% sure but don't have absolute knowledge, you're self-deluded. If you're just a little less than 100% sure, you're probably under the influence of a pernicious cognitive bias: underestimation of uncertainty, also known as the Overconfidence Effect.
The Overconfidence Effect is a pervasive and well-documented human bias where the level of certainty in one's beliefs is usually much higher than the accuracy of those beliefs. It has been studied by asking people to answer questions (e.g., the spelling of difficult words) and then asking how sure they are that each answer is correct. Those studies found that when people were "100% sure", they were wrong approximately 20% of the time. When 99% sure, they were wrong 40% of the time, and when 90% sure, they were wrong approximately 50% of the time. That should put human certainty into perspective!
I think even 90% is unreasonable for theological beliefs, despite the high certainty that so many believers and atheists seem to have. 90% certainty implies 90% probability. Starting with the principle of indifference, a 90% probability that God exists (or doesn't exist) would require very strong evidence. Though I think there is solid evidence for the God of the Bible, I haven't seen enough for 90% certainty either way. Certainty that a particular religion or systematic theology is the "correct one" would require that plus a lot more. I haven't seen it yet, but that's no reason not to have faith in whichever of the available possibilities is most probable according to the evidence we do have.
I've heard many times, "If God wants us to believe in him, why didn't he give us more evidence?" I think the question totally misunderstands faith and the Bible's message about it. Faith does not mean believing God exists. The God of the Bible really didn't seem to care that people believed he existed. What mattered was whether they trusted his promises and lived in a way that reflected confidence (despite uncertainty) that he would be faithful to those promises. That's very different from the alternatives, such as the faith of atheism (i.e., living in a way that reflects confidence that there is no God and thus no divine promises to be fulfilled). If there was sufficient evidence (or philosophical arguments) to know without any doubt, such choices would mean very little.
Whether we admit it or not, we all have faith because we all make decisions amid uncertainty. Uncertainty isn't such a bad thing. It makes us more humble about our beliefs and more respectful of the beliefs of others, which makes us more open to the truth, in the (very likely) case that we aren't totally correct about everything we believe. Uncertainty also makes faith a lot more meaningful.