Doubt is not unbelief. But it can become unbelief. That basic principle should guide our reflections on this important issue. Doubt is natural within faith. It comes about because of our human weakness and frailty. We lack the confidence to trust fully in God and long for certainty in all matters of faith. But absolute certainty is hard to come by. You can be sure that 2 + 2 = 4, but is that going to change your life? Is that going to give you a reason to live and hope in the face of death? And it isn't just Christians who are in this situation. The atheist's belief that there is no God is just as much a matter of faith as your belief that there is! Doubt also comes about through our lack of humility. All of us are tempted to believe that because we haven't got the answers to the hard questions of faith, then there aren't any answers to those questions.
We need to learn to be relaxed about doubt. Doubt is like an attention-seeking child. The more attention you pay to it, the more attention it demands. By worrying about your doubts, you get locked into a vicious cycle of uncertainty.
So how does doubt become unbelief? Unbelief is the decision to live your life as if there is no God. It is a deliberate decision to reject Jesus Christ and all that He stands for. But doubt is something quite different. Doubt arises within the context of faith. It is a wistful longing to be sure of the things in which we trust. But it is not, and need not, be a problem. Just because I can't prove my faith in God doesn't mean that it is wrong.
But unbelief can creep in during those moments of doubt. How? Think of your faith as a lifeline to God. Think of it as being like an umbilical cord, linking you to God and providing a channel through which His life-giving grace can reach you. Sever that link and faith will wither, just as a branch which is broken off a vine shrivels and dies (John 15:1-6). Have you read C.S. Lewis' Screwtape Letters? If not, there is a real treat in store for you. But if you have, you will know how Lewis points out that Satan uses ploy after ploy to try to get Christians to break their links with God. Doubt is one of those ploys.
Think through what will happen if Satan can manage to get you obsessed with your doubts. You'll start becoming introverted, as you become preoccupied with your doubts. You'll look inwards, at yourself and your state of mind. And you will stop looking outwards, away from yourself and toward the promises of God, confirmed and sealed through the death and resurrection of Christ. The more you worry about your doubts, the less you will look to God. Gradually, those vital links with the life-giving grace of God will wither—and your spiritual life will wither and shrivel. Doubt will become unbelief—because you allowed it to. Feed your doubts and your faith will starve—but feed your faith, and your doubts will starve. Doubt initially becomes a problem, and finally becomes unbelief, if, and only if, you allow it.
Unbelief thus comes about through several possible routes. First, through an unrealistic attitude to faith. If you believe that you can, or need to, know everything with absolute certainty, your faith will be in difficulties very soon. But faith isn't like that! Faith is about being willing to live, trusting in the existence and promises of God, knowing that one day, that existence and those promises will be totally vindicated. But for the moment, we walk by faith, not by sight.
Second, unbelief may come through a morbid preoccupation with doubt, by which you become so obsessed with your own mental states and feelings that God is shut out of your life. Give Him some breaks! Look outward, not inward! Look to the promises of God; savor them; accept them. Stop allowing your doubts to dominate your life. Doubt, seen properly, is just the darker side of faith; rediscover the "sunnier side of doubt" (Tennyson)—the joy of faith itself.
And third, unbelief may come through an immature faith—a faith which refuses to grow up. A weak faith is a vulnerable faith. The process of maturing as a Christian involves deepening our understanding of what we believe. As we grow older, we are meant to deepen our understanding of our faith. The things that bothered us when we were young in faith don't bother us quite so much. In fact, if I might speak from personal experience, I now realize that most of my own early doubts simply reflected my inadequate understanding of my faith. As I grew older, I grew wiser—through reading, thinking, and listening to or reading wise Christians. Reinforce faith with understanding, in much the same way as you would reinforce concrete with steel. Together, they can withstand far greater stress than they could ever withstand on their own.
When does doubt become unbelief? Answer: When you let it. When you cling to unrealistic ideas about faith, when you get hopelessly preoccupied with the doubts that are a natural part of the Christian life, or when you fail to allow your faith to grow. These pitfalls can all be avoided. Don't feel ashamed about your doubts. Talking them through with older and wiser Christians can be a vital safety valve which stops a head of doubting steam from building up—a head of steam which could eventually lead from normal doubt to the hopelessness of unbelief.