The problem comes from equating certainty with demonstrable proof. A Christian can be "sure of what we hope for and certain of what we do not see" (Heb 11:1); that is, a Christian can have certainty of their faith, but only by not having it in him/herself but by faith in Christ. In other words, as a Christian I am certain of something I absolutely cannot prove to others or even to myself. It is Christ that proves himself, both to me and to those to whom I testify about him. Christ has and does prove himself to me by constantly reforming my life and thought (there is a lot to reform; its taking a while) and therefore I must speak of him. But I cannot repeat to others the proof of himself he has given to me; he must prove himself to them also. This is humiliating to me.
When I testify not only of the existence of God in Christ, but of his infinite goodness and faithfulness in him, I constantly put myself up to questioning. It is my impulse as a man, an "intellectual", an American, a human, for crying out loud, to answer these questions with overwhelming logic and clarity. But this can only fail, and failure is humiliating. We make huge claims as Christians. Can we really offer no proof, no evidence? Of course, we can point to the historical evidence and the evidence of our own changed (however minimally) lives, but certainly the person questioning cannot find these things, especially on our frail lips, sufficient evidence to become a Christian. But this usually only goads me on to pile the rhetoric higher, push the logic farther, make the case, make the sale. I'm usually totally unaware of the point where it stops being about Christ and becomes about me winning the argument. Someone would have to be an idiot to become a Christian because I won an argument, but this kind of apologetics presupposes that they should. But what CAN I do? I must give an answer for the hope that I have. How do I do that without making it about my superior argumentative skills? I am learning that the answer is to constantly and ruthlessly point away from myself to Christ.
Christ must always be the justification for my faith in him; it cannot reside in me or be put into my control. This is a part of justification by grace alone; all of us can only know Christ by the grace of God, not by reason alone or empirical observation alone. Thus I can only answer everyone who asks me to give the reason for the hope that I have by setting apart Christ as Lord in my heart, by pointing away from myself, and all of my logic and certainty, to him; he is the reason. Only he can be the reason for others to hope as well. I cannot give them faith; I can only point to the grace that has given me faith and gives it freely to all who ask in Christ's name.
When I understand this, gentleness and respect for others is a necessary byproduct. I assume this is coming; God isn't through with me yet.