There are a number of possible answers the question of why I am a Christian. One is that I was born a Christian. But that answer just pushes the question back: why, then, do I remain a Christian? I suppose the answer is that I have come to believe Christianity is right. By "right" I mean that I believe it describes the cosmos accurately. If one were to come up with a core list of propositions that describe Christian belief essentially (maybe the three great Creeds or soemthing), then all of those propositions would be true.
Thinking Christianity to be RIGHT assumes that Christianity is a metaphysical scheme; and in a sense it is. But more than that, I think evangelicals are onto something important when they say that Christianity is a RELATIONSHIP. In a very important sense, Christianity is essentially a relationship of the individual creature with God in Christ. And as with any relationship, the indiidual Christian's relationship with God in Christ ought to have fruits, or consequences, in the life of the Christian. That is certainly true of my life. That is, my life, my activities, my outlook, the little things I do each day, are all diffferent in virtue of my being a Christian, of being in a relationship with God in Christ. Moreover, the DIFFERENCE in my life reenforces the act of intellective belief. And the stronger the faith, the more I feel I am able to see how different my life is in virtue of the Christianity as a relationship. In an important sense, therefore, I AM a Christian -- or maybe I have remained a Christian -- because being a Christian has made a difference in my life.