It might be said that...to have a conviction is to be subjectively certain—to feel with great confidence that one’s beliefs, values, and aims are correct. Imagine, for example, a theist who admitted that she is not certain that God exists but believes in God nevertheless. We could take her to mean that she has no sufficient evidence or proof for God’s existence, but rather believes it on faith. And we might then suggest that to believe something on faith is to be subjectively certain about it—to feel with great confidence that this belief is correct or true. But this will not do. People struggle with their faith, question it, seek to better understand it—and also struggle to bring their lives and other beliefs into conformity with their faith (or convictions).
If I take something on faith, then I resolve myself to accept it, to go along with it, to let it shape me and to shape my actions around it. In none of this is a requirement that I feel certain that what I am committing myself to is correct (or true). It is possible that I have reservations about going forward. I might have doubts, but my doubts about turning back might be even greater, and I might find myself forced by my situation either to press forward or retreat. To press forward in faith, or with conviction, is to devote myself to that path—to give myself to it. Perhaps I can withdraw later if things aren’t working out. But if I continue to think that, then I am not living with faith or conviction. It is not a lack of certainty that destroys conviction, but rather a lack of devotion.
A person might show such conviction—and devotion—in a relationship such as a marriage. A person might not know, let alone feel certain, that his spouse is his soul mate. One might not put much stock in such fine phrases. At the same time, one can be committed to his or her matrimonial vows, committed to cultivating a relationship in which continued love is possible. Such a commitment is not a prediction of what will happen, that the marriage will not fall apart (or at least that if it does, this person won’t be the one responsible for it). Thus, such a person—without showing any lack of conviction with regard to the depth of his devotion—could say, “I am not certain that things will work out between us.” This is not, of course, the sort of thing to be said on one’s wedding day. It could, however, be said in full seriousness—to a friend, perhaps—during a serious marital disagreement. “Do you believe that things will work out?” “I don’t know.” Such a confession of ignorance and uncertainty has nothing to do with whether one is still devoted to the marriage. Being committed to making it work out—to the extent that this is in one’s control—is not the same thing as being committed to the proposition that it will work out. Of course, if one has no hope that it will work out, then there would be little point in being committed to making it work. Thus, there is a connection between conviction and devotion, and between devotion and hope. If certainty has any role to play here, it is simply that one must not feel subjectively certain that the relationship is hopeless. When that is gone, then one had better get a priest or a lawyer.
(I wrote this before actually looking back over p. 168ff in Williams' Ethics and the Limits of Philosophy, to which Tommi had quite helpfully referred me. There's definitely some overlap here with Williams, and I imagine I will take up some of what Williams says around p. 169 about conviction being somehow "inescapable" soon, though some of what I said here is a start on that. For what it's worth, I take a similar, though I don't think as well-articulated, position on conviction and subjective certainty around the second page of my forthcoming paper "Moral Conviction.")