Reason is infirm in that it cannot establish anything definitively. It cannot even prove that doubting is the way to truth, "that it is certain that we ought to be in doubt." (Pyrrho entry, Bayle's Dictionary, tr. Popkin, p. 205) But, pace Pierre Bayle, the merely subjective certitude of faith is no solution either! Recoiling from the labyrinth into which unaided human reason loses itself, Bayle writes:
It seems therefore that this unfortunate state [the one brought about by the infirmity of reason] is the most proper one of all for convincing us that our reason is a path that leads us astray since, when it displays itself with the greatest subtlety, it plunges us into such an abyss. The natural conclusion of this ought to be to renounce this guide and to implore the cause of all things to give us a better one. This is a great step toward the Christian religion; for it requires that we look to God for knowledge of what we ought to believe and what we ought to do, and that we enslave our understanding to the obeisance of faith. If a man is convinced that nothing good is to be expected from his philosophical inquiries, he will be more disposed to pray to God to persuade him of the truths that ought to be believed than if he flatters himself that he might succeed by reasoning and disputing. A man is therefore happily disposed toward faith when he knows how defective reason is. (206, emphasis added)
Now how is this a solution to the alleged infirmity of reason? A Christian fideist, acquiescing in pure blind (purblind?) faith, accepts the Trinity while a Muslim fideist, equally subjectively certain of his faith, rejects the Trinity while intoning that God is one. Blind conviction butts up against blind conviction of the opposite kind and all too often strife and bloodshed is the upshot.
Admittedly, reason is weak and inconclusive. But fideistic faith is blind: the certitude it provides is merely subjective. For if the Christian's subjective certitude of the Trinity were also objective, then the same would hold for the Muslim's subjective certitude of the opposite, with the result that one and the same proposition would be both objectively true and objectively not true. To avoid this result one would have to throw out the Law of Non-Contradiction. But then one would have taken one step too far.
The dogmatic Christian will claim that his subjective certainty is also objective; but the dogmatic Muslim will do the same. Obviously, they can't both be right. So one must be wrong. Each will call the other wrong. But neither will be justified in doing so.
Although there is scholarly debate as to what exactly Pierre Bayle's ultimate philosophical position is, the following argument can be attributed to him (cf. Richard H. Popkin, "Pierre Bayle's Place in 17th Century Scepticism" in Paul Dibon, ed. Pierre Bayle: Le Philosophe De Rotterdam, Elsevier, 1959, pp. 1-19):
1. Reason is too weak and confused to discover the truth about the world and how we should live in it.
Therefore
2. One must rely on faith as the guide to, and divine revelation as the measure of, the truth about the world and how we should live in it.
This is a non sequitur for a couple of reasons. First, one response to the supposed truth of (1) might be a universal epoché (εποχη) of all contention-inspiring beliefs and a resolve to stick to the mundane and follow the customs of one's time and place, and in the relaxed manner in which worldly people do this: one goes along to get along without getting too worked up over anything. Among meat-eaters one eats meat, among vegetarians vegetables only. Above all, one does not dogmatize about the evils of factory farms, the health dangers of red meat, etc. Second, the argument gives one no reason to accept the Christian faith and the Christian revelation as opposed to, say, the Muslim faith and revelation. (See the Bayle quotation above.)
My tentative conclusion, then, is that the infirmity of reason, though a fact, does not straightaway license any embrace of fideism which, for present purposes, can be characterized roughly as reliance on pure blind faith in matters of religion. (Kierkegaard (1813-1855) is a good 19th century example of a fideist.) Fideism comes in different varieties. There is Wittgensteinian fideism, to mention just one variety.
What are our options? Perhaps only these:
A. Rationalism: Put your trust in reason to deliver truths about ultimates and ignore the considerations of Sextus Empiricus, Nagarjuna, Bayle, Kant, and a host of others that point to the infirmity of reason.
B. Fideism: Put your trust in blind faith. Submit, obey, enslave your reason to what purports to be revealed truth while ignoring the fact that what counts as revealed truth varies from religion to religion, and within a religion from sect to sect.
C. Skepticism: Suspend belief on all issues that transcend the mundane if not on all beliefs, period. Don't trouble your head over whether God is or is not tripersonal. Stick to what appears. And don't say, 'The tea is sweet'; say, 'The tea appears sweet.' (If you say that the tea is sweet, you invite contradiction by an irascible table-mate.)
D. Reasoned Faith: Avoiding each of the foregoing options, one formulates one's beliefs carefully and holds them tentatively. One does not abandon them lightly, but neither does one fail to revisit and revise them. Doxastic examination is ongoing at least for the length of one's tenure here below. One exploits the fruitful tension of Athens and Jerusalem, philosophy and religion, reason and faith, playing them off against each other and using each to chasten the other.
I recommend (D). Or are there other options?