What, then, should a Christian be afraid of regarding God? Think of it like this. Imagine that you are suddenly introduced to some person you have always admired enormously - perhaps someone you have hero-worshipped. You reach out to shake her hand and suddenly it hits you. You can't believe you are actually meeting her. You discover to your embarrassment that you are trembling and sweating, and when you try to speak, you are out of breath. What is going on? You are not afraid of being hurt, or punished. Rather, you are genuinely afraid of doing something stupid or saying something that is inappropriate to the person and the occasion. Your joyful admiration has a fearful aspect to it. You are in awe, and therefore you don't want to mess up.
... We could say that fear of punishment is a self-absorbed kind of fear. It happens to people wrapped up in themselves. Those who believe the gospel - who believe that they are the recipients of undeserved but unshakable grace - grow in a paradoxically loving yet joyful fear. Because of unutterable love and joy in God, we tremble with the privilege of being in his presence and with an intense longing to honor him when we are there. We are deeply afraid of grieving him. To put it another way - you would be quite afraid if someone put a beautiful, priceless, ancient Ming dynasty vase in your hands. You wouldn't be trembling with fear about the vase hurting you, but about you hurting it. Of course, we can't really harm God, but a Christian should be intensely concerned not to grieve or dishonor the one who is so glorious and who did so much for us.
Tim Keller, Prayer, pp 98-99