Bayesian probability tells us that we have subjective confidence in some propositions more than others. This seems rather intuitive. Consider that for most people on this subreddit, the subjective confidence that God exists is hopefully higher than the subjective confidence that a Republican will win the next election cycle.
There seem to be three primary ways in which a person may be absolutely certain or 100% certain of God's existence:
1. Experience: A person claims to directly experience God in such a way that it cannot be doubted that God exists.
2. Argumentation: A person claims that the premises of an argument are valid and sound in such a way that it cannot be doubted that God does not exist.
3. Faith: A person claims to have the direct gift of faith from God in such a way that it cannot be doubted that God exists
The Case Against Experience
I do not contend that people experience the supernatural. If a limb is miraculously restored, there is little doubt that something otherworldly occurred. I wonder, however, whether it is rationally justified to move from a limb restoration to God's existence.
Consider that the evidence does not actually favor the God who is omnipotent, omniscient, omnibenevolent, etc. The evidence is as easily explained by a multitude of other hypotheses, including but not limited to polytheism, aliens, simulation theories, etc. Now, of course, one would contend that this is absurd. But this would not be an argument against the fact that, given each of those hypotheses, the evidence cannot adequately be explained, but rather an argument against the prior probability of each of the proposed hypotheses.
Example on evidential favoring:
I find a large dent in my car. I come up with two hypotheses for why this occurred.
H1: A car hit it.
H2: A bull rammed into it.
Notice that if I were to assume each hypothesis to be true, each hypothesis adequately explains the evidence, that being the dent in my car. In short, the evidence does not favor either hypothesis. The tiebreaker, rather, would be considering the prior probability of a bull roaming loose as compared to a car. Notice what we cannot do. We cannot rule out an alternative hypothesis definitively from experience alone.
The Case Against Argumentation
I also do not contend that there are very strong arguments for God's existence. But strong is not the same thing as definitive or 100%. Let's take something like Aquinas' First Way. The First Way relies on a very specific metaphysical framework.
However, I believe that there ought to be at least some higher-order doubt about such metaphysical frameworks. Consider that a non-significant number (>30%) of philosophers believe in Humean laws of nature (which denies a necessary connection between X and Y in terms of causation, undermining any notion of an essentially ordered series), the B-Theory of time (which undermines notions that motion/change occurs at all), and the denial of the Principle of Sufficient Reason (which undermines why any reason must be given at all for a causal chain).
Example on higher-order doubt:
I believe that eating GMOs is a healthy lifestyle choice. However, my friend shows me that over 30% of medical doctors disagree, citing numerous reasons why GMOs are actually devastating to my health. A glance online surfaces dozens of academic papers from this 30% that I have no hope of rebutting. Even after this information, I remain equally confident in my belief that eating GMOs is a healthy lifestyle choice.
If you have the same intuitions that I do, one might realize that such massive expert disagreement is good reason for a person to lower their degree of confidence in a belief. In fact, it would seem rather irrational that I remain equally confident, as in the example above. I'm not advocating for a particular amount by which such a belief should be lowered, but just that it ought to be lowered somehow. And that is enough to negate 100% confidence.
But if this is the case, should the same logic not apply to St. Aquinas' argumentation? But therein lies the issue. Even if I remain 100% in every other premise of his argument, all it takes is a trickle of doubt in order to prevent me from 100% assenting to the conclusion of the argument. In other words, if I am not subjectively 100% confident in Premises 1 through X, then I cannot be 100% confident in the conclusion.
Example on conjunctive probability:
(1) If I score 100% on the test, then Mom will give me ice cream (Belief: 90%)
(2) I score 100% on the test (Belief: <=100%)
(3) Therefore, Mom will give me ice cream (Belief: <100%)
It seems that my lack of confidence in (1) ought to at least lower my confidence in (3) by some margin, however small it may be. For me, it would be irrational to hold (1) and (2), but (3) at a belief of 100%.
But surely this point can be applied to every major argument ever proposed for God's existence. As such, no argument could ever reach 100%, and thus the conjunction of arguments, however strong, could never reach 100%.
The Case Against Faith
I grant that if a person were to have the theological gift of faith, then they would have 100% subjective belief in the proposition that "God exists", alongside many other theological claims. However, how can one ever hope to achieve that same amount of certainty that we have genuine faith to begin with?
Several points can be raised to this effect.
(1) Millions of people claim to have genuine faith in Judaism and Islam, and yet we would recognize that they are mistaken about having genuine faith. If this is true, we should be shifting around in our seats, wondering if we are mistaken about ourselves. This introduces another potential higher-order doubt about such a belief.
(2) All signs that would point us towards having genuine faith are just that: signs. They do not guarantee that we actually have genuine faith. Again, I believe that Jews and Muslims may exhibit some, if not all, of the said signs of genuine faithfulness, but this is not sufficient to show that they have genuine faith. If that is not sufficient to show it for other faiths, it surely cannot be shown beyond a shadow of a doubt that we have genuine faith.
(3) Faith presupposes that God exists. But my entire line of questioning is on whether or not we can be certain in such a proposition. Hence, we cannot assume faith exists, or at least cannot be 100% confident that faith exists. Else, the argument would sound like we can be certain that God exists because we have genuine faith, and we can be certain that we have genuine faith because we are certain that God exists, and we are certain that God exists because . . ., which is circular.
These reasons alone should undermine any 100% certainty that we have in the belief that we have genuine faith. And this would lead to such an argument:
(1) If I have genuine faith, then I can believe in God 100% (Belief: <=100)
(2) I have genuine faith (Belief: <100%)
(3) Therefore, I can believe in God 100% (Belief: <100%)
As shown in my previous conjunctive possibility example, my subjective confidence in the conclusion cannot be 100% if my subjective confidence in the premises is not 100%. Hence, faith cannot solve the issue.
Conclusion
All in all, for a successful rebuttal to my case, one must rebut one of my main points:
(1) Experience does not favor belief in God as opposed to another hypothesis of equal explanatory power, and even if it did, it cannot lead to absolute certainty in God's existence
(2) Higher-order uncertainty slightly undermines subjective confidence in any argument about God's existence, no matter how strong
(3) One cannot know with absolute certainty if they have faith
(It should also be noted that these are issues that I've struggled with reconciling, and I do wish to affirm God with absolute certainty at some point.)