In God's casino

1. The Puritans, or at least some Puritans, championed an infallibilist religious epistemology which became enshrined in the Westminster Confession:

We may be moved and induced by the testimony of the Church to an high and reverent esteem of the Holy Scripture. And the heavenliness of the matter, the efficacy of the doctrine, the majesty of the style, the consent of all the parts, the scope of the whole (which is, to give all glory to God), the full discovery it makes of the only way of man's salvation, the many other incomparable excellencies, and the entire perfection thereof, are arguments whereby it does abundantly evidence itself to be the Word of God: yet notwithstanding, our full persuasion and assurance of the infallible truth and divine authority thereof, is from the inward work of the Holy Spirit bearing witness by and with the Word in our hearts (WCF 1:5).

This certainty is not a bare conjectural and probable persuasion grounded upon a fallible hope; but an infallible assurance of faith founded upon the divine truth of the promises of salvation, the inward evidence of those graces unto which these promises are made, the testimony of the Spirit of adoption witnessing with our spirits that we are the children of God, which Spirit is the earnest of our inheritance, whereby we are sealed to the day of redemption (WCF 18.2).

I believe John Owen tries to unpack an infallibilist epistemology, but I'm not going to discuss that. Let's consider some distinctions and definitions:

There are various kinds of certainty. A belief is psychologically certain when the subject who has it is supremely convinced of its truth.

A second kind of certainty is epistemic. Roughly characterized, a belief is certain in this sense when it has the highest possible epistemic status.

Some philosophers also make use of the notion of moral certainty (see Markie 1986). For example, in the Latin version of Part IV of the Principles of Philosophy, Descartes says that “some things are considered as morally certain, that is, as having sufficient certainty for application to ordinary life, even though they may be uncertain in relation to the absolute power of God” (PW 1, pp. 289-90). Thus characterized, moral certainty appears to be epistemic in nature, though it is a lesser status than epistemic certainty.

Certainty is often explicated in terms of indubitability.

According to a second conception, a subject's belief is certain just in case it could not have been mistaken—i.e., false.

According to a third conception of certainty, a subject's belief that p is certain when it is justified in the highest degree.

https://plato.stanford.edu/entries/certainty/

Of course, it would be somewhat anachronistic to apply this taxonomy to the Westminster Confession. Still, we might ask how to classify "infallible assurance" according to that taxonomy? Seems like it dovetails with all the variations: psychological certainty, epistemic certainty, indubitability, justified in the highest degree, and unable to have been mistaken. But is that true?

2. Among other things, the Westminster Confession links infallible assurance to the witness of the Spirit. One way of construing that claim is that the witness of the Spirit bridges the gap between evidence and assurance. I don't know if that's what the Westminster Divines had in mind, and since the Westminster Assembly was comprised of many individuals, there may have been a variety of views, even if they share a family resemblance.

Now it might be objected that if the witness of the Spirit is a makeweight which confers a degree of assurance that outstrips the evidence, then that's fideistic. But is it? Surely God is capable of inducing certitude. If, moreover, that mental state corresponds to objective truth, then it seems to be warranted. Indeed, it was generated by a reliable belief-forming process. So it seems to meet the condition of epistemic certainty, and goes beyond that, since it could not have been mistaken. 

3. That said, is it necessary to raise the bar that high? If Christianity is true, then it's 100% true. Now suppose, for argument's sake, that I have 60% confidence in Christianity. Although I think it's silly to mathematically quantify degrees of certainty, let's do it for illustrative purposes. And suppose 60% confidence suffices for saving faith. That means 60% confidence will get me 100% salvation. Epistemologically, it's only 60% certainty, but ontologically, it's 100% heaven! Sounds like a deal to me!

Moreover, even though I expressed the relation in artificial terms, yet if we're saved by grace, then it's not as if salvation depends on our ability to muster 100% certainty. Or if it did, and God intends to save someone, he will grant them 100% certainty.

4. Put another way, above a necesary threshold, the level of certainty doesn't affect the outcome. The promise of salvation isn't adjustable to the degree of certainty. My degree of certainty can't change reality. The ontology of the Christian faith is independent the psychology and epistemology of faith. Certainty doesn't make it any truer while doubt doesn't make it any less true. 

5. The main thing is whether we can know enough to make the right choice between Christianity and its rivals. That doesn't require absolute certainty. 

6. Finally, there's some tension between faith and certainty. There's a sense in which faith is meant to be a gamble. Where the nature of faith requires an element of uncertainty. 

For faith involves trusting another. It isn't direct knowledge, but letting someone else be your eyes and ears. And psychologically speaking, that doesn't feel as certain as seeing something for yourself. Indeed, it's supposed to be different in that regard. That's what makes it faith. If you could see it for yourself, there'd be no need to exercise faith. No need to put your trust in someone else. 

Take the prospect of dying. Most of us only die once. Most of us don't have a near-death-experience. And even if we did, that's not the same thing as Lazarus returning to life four days later. 

Most of us have no direct experience of what lies on the other side of the grave. We don't know from firsthand experience what awaits us. We don't know from firsthand experience if there's anything on the other side (apparitions of the dead excepted). 

If there is no afterlife, we won't know what hit us. And if we're hellbound, it's too late to prepare for death. 

This parallels risk assessment, where there are two variables, viz. a minor risk of major harm or major risk of minor harm. So even if you had a very high level of confidence, you might still be nervous if you have everything to lose in the unlikely event that you're mistaken. 

I don't think it's inherently unholy for Christians to have some anxieties about death, where you must put everything on the line, for faith is meant to be a kind of gamble–where you hazard everything for God. The element of uncertainty is what makes it an act of total devotion. Psychologically costly. You don't hedge your bets. You leave nothing in reserve. You put all your chips on the table, both despite and because of what's at stake. 

Mind you, God does things to make that easier. Death is unavoidable. And the evidence for Christianity is decisively superior to the competition. In that sense, it's a low-risk gambit. But it's still suspenseful.

To take a comparison, suppose your wife and kids are abducted. The kidnapper demands a ransom that's beyond your means. However, you make an arrangement with a cardsharp at the local casino. He will deal you winning cards in exchange for a cut of the winnings. That way you can raise enough money to pay the ransom.

Yet even though the deck is stacked in your favor, you still feel jittery was you wait to see the next card, and a sense of relief as dealer comes through, for there's so much on the line, and you have no direct control over the outcome. You're entirely dependent on someone else to act on your behalf in your vital interests. That forces you to live by faith.