This is a question that gets tossed around a lot in debates about religion.
Sometimes a Christian asks an atheist this, with the implication being that in order to reject Christianity and put one’s immortal soul at risk for eternity, one ought to be very sure that it is false. I always cringe when I hear it in this context, for two reasons. First, my understanding of hell and who goes where differs from the conventional view; issues of salvation and the afterlife are simply not as clear-cut as many Christians present them to be in arguments or tracts. I often wonder whether those making such arguments genuinely believe them or whether they think that a forceful, simplified view is more likely to win converts. They are probably mistaken in either case. Second, the idea that certainty is required to risk one’s soul on rejecting Christianity is a simplistic presentation of Pascal’s wager. As I discuss here, I think that in a more general formulation, Pascal’s wager is an insightful argument against the carpe diem attitude, but as a direct argument for Christianity it doesn’t work.
Sometimes, it is the atheist who asks the Christian “how can you be so sure that you’re right?”. In some cases, the implication is that in order to believe in anything supernatural, one must have solid proof. Atheism is assumed to have a privileged position; the burden of proof is conveniently on the other party. From there, the discussion usually degenerates, with the Christian responding “prove to me that God does not exist”. Within a few minutes, the Flying Spaghetti Monster is usually tossed in for good measure.
In other cases, of course, sometimes the question is being asked rather straightforwardly. Christians, myself included, often use language that implies certainty about beliefs. Partly this is due to the linguistic heritage of the New Testament. Those who witnessed Christ’s miracles (such as Peter, John, James) or otherwise encountered Him post-Resurrection personally and directly (such as Paul) testified out of their own certainty, and whenever we quote them approvingly (as when we celebrate using texts like 2 Cor 5:1-9), we reflect that certainty to our listeners. If pressed to be specific, however, I think most of us (myself included) should admit to having lingering doubts, since (for most of us anyways) our knowledge of Christ is second-hand. I can judge the Gospels to be truth rather than fraud or mass hallucination, but I can never be 100% (or 99% for that matter) certain, unless God choose to intervene in my life in a manner that removes all doubt. And if He does, it wouldn’t fully convince anyone but me.
One might wonder why a God who proclaims faith to be a virtue and doubt to be a sin would put most of us in a situation of uncertainty; I have done my best to address that question here. (And the related question about why prayers, for wisdom or certainty amongst other things, can long go unanswered is addressed here.) Nevertheless, given the situation, it is required of Christians to encourage one another. A pastor who mused about his personal doubts in a Sunday sermon would not be doing any favors to the family of a recently deceased loved one, or to a man struggling with a great temptation, or to a hard-working, underappreciated volunteer or caregiver. However, the pastor who boldly strengthens his flock and sends them out into the world as a force for love and goodness and mercy might not be the ideal candidate for engaging with skeptics. The same dilemma faced by a pastor is true of every mother, father, and friend. How does one strike the perfect balance between encouraging one’s neighbors (and oneself) and honestly acknowledging one’s own struggles with doubt? If anyone out there figures it out, please do drop me an email. I would imagine it’s a delicate effort. If a Christian is trying to win over the skeptic to his point of view, it may seem unwise to talk about one’s own doubts. It’s rare to hear, for example, a candidate for office talk about his opponent’s strengths, or a business talk about the best features of their competitors’ products. Nonetheless, I think that a Christian who proclaims certainty in what he or she preaches risks being seen either as dishonest, or as so intellectually naïve as not to understand the skeptic’s concerns.
I’ve discussed how the question sounds coming from a Christian or an atheist, but of course there is another common source, that of the agnostic. By agnostic, I don’t just mean those who label themselves as such or who fit a technical definition. An agnostic may sport a conventional religious label that doubles as a cultural or ethnic one (Catholic or Jewish), or might eschew such labels altogether. The key feature of the agnostic is not, in my view, their doubt, but their lack of commitment. The agnostic does not subscribe to strong beliefs about God and therefore enjoys the freedom to make balanced, politically correct statements such as “I’m not a religious man, but….”, or “I don’t know if there’s a God or not, but if there is….”, or (as seems to be increasingly prevalent) refer to “karma” without a context in Buddhism or other Eastern religions.
The agnostic is usually annoyed listening to atheists and Christians argue their differences, and asks “How can you be so sure that you’re right?” to both parties. He or she may realize (correctly, I think) that absolute certainty is not possible in such matters (this side of the grave, anyways). Furthermore, as noted earlier, sometimes the Christian (or the atheist) is taking a hard line on the certainty of their claims, being in the heat of battle as it were. The agnostic may picture himself as simply being honest about the obvious truth that questions about God are beyond our ability to resolve through argument, and may picture Christian and atheist alike as wasting time and annoying everyone else. Nevertheless, the implicit assumption seems to be that without certainty, one should not take a position. It’s the equivalent to sitting at home in a momentous election with two stark choices, because you aren’t entirely happy with either candidate. I once fell prey to that sentiment and sat out a Congressional election myself, and am not proud of it.
But whereas one person can almost never swing an election, one person can make a difference with their life, either through good, evil, or indifference. All three are choices. There is good evidence in favor of the existence of God and the truth of Christ’s message, and I believe in both, but I do not feel absolutely certain in my beliefs (and the more specific the particular point of truth being considered, the less confident I feel that I have the right answer). It is not within my power to choose to feel certain; if God wants to give me that confidence at some point I would welcome it but that is His call. I can, however, choose what to do with my life. I can choose to commit to Christ and call myself His servant. And that is what I have done.
In conclusion, it seems that Christian, atheist, and agnostic alike are often asking the wrong question here. “How can you be so sure that you’re right?” puts a combative tone into a discussion of questions of ultimate truth that ought not be debated for sport, but treated with the seriousness they deserve. Perhaps a more productive, positive question to ask would be “why do you believe what you believe?”.