In Surprised by Joy, C. S. Lewis describes his journey by stages from materialism, to idealism, to theism, and finally to Christianity. The account is too long to reproduce here; but I quote below what I find to be some of his most powerful observations along the way.
The odd thing was that before God closed in on me, I was in fact offered what now appears a moment of wholly free choice. In a sense. I was going up to Headington Hill on the top of a bus. Without words and (I think) almost without images, a fact about myself was somehow presented to me. I became aware that I was holding something at bay, or shutting something out. Or, if you like, that I was wearing some stiff clothing, like corsets, or even a suit of armor, as if I were a lobster. I felt myself being, there and then, given a free choice. I could open the door or keep it shut; I could unbuckle the armor or keep it on. Neither choice was presented as a duty; no threat or promise was attached to either, though I knew that to open the door or to take off the corset meant the incalculable. The choice appeared to be momentous but it was also strangely unemotional. I was moved by no desires or fears. In a sense I was not moved by anything. I chose to open, to unbuckle, to loosen the rein. I say, “I chose,” yet it did not really seem possible to do the opposite. On the other hand, I was aware of no motives. You could argue that I was not a free agent, but I am more inclined to think that this came nearer to being a perfectly free act than most that I have ever done (p. 224).
I was allowed to play at philosophy no longer. It might, as I say, still be true that my “Spirit” [i.e., a merely philosophical conception of God] differed in some way from “the God of popular religion.” My Adversary waived the point. It sank into utter unimportance. He would not argue about it. He only said, “I am the Lord”; “I am that I am”; “I am.”
People who are naturally religious find difficulty in understanding the horror of such a revelation. Amiable agnostics will talk cheerfully about “man’s search for God.” To me, as I then was, they might as well have talked about the mouse’s search for the cat (p. 227).
You must picture me alone in that room in Magdalen, night after night, feeling, whenever my mind lifted even for a second from my work, the steady, unrelenting approach of Him whom I so earnestly desired not to meet. That which I greatly feared had at last come upon me. In the trinity Term of 1929 I gave in, and admitted that God was God, and knelt and prayed: perhaps, that night, the most dejected and reluctant convert in all England. I did not then see what is now the most shining and obvious thing; the Divine humility which will accept a convert even on such terms. The Prodigal Son at least walked home on his own feet. But who can duly adore that Love which will open the high gates to a prodigal who is brought in kicking, struggling, resentful, and darting his eyes in every direction for a chance of escape? The words compelle intrare, compel them to come in, have been so abused by wicked men that we shudder at them; but, properly understood, they plumb the depth of the Divine mercy. The hardness of God is kinder than the softness of men, and His compulsion is our liberation (pp. 228-229).
He describes all this as his conversion “to Theism, pure and simple, not to Christianity.”
My conversion involved as yet no belief in a future life. I now number it among my greatest mercies that I was permitted for several months, perhaps for a year, to know God and to attempt obedience without even raising that question… I had been brought up to believe that goodness was goodness only if it were disinterested, and that any hope of reward or fear of punishment contaminated the will. If I was wrong in this (the question is really much more complicated than I then perceived) my error was most tenderly allowed for… God was to be obeyed simply because he was God. Long since, through the gods of Asgard, and later through the notion of the Absolute, He had taught me how a thing can be revered not for what it can do to us but for what it is in itself. That is why, though it was a terror, it was no surprise to learn that God is to be obeyed because of what He is in Himself. If you ask why we should obey God, in the last resort the answer is, “I am” (pp. 231-232).
Every step I had taken, from the Absolute to “Spirit” and from “Spirit” to “God,” had been a step toward the more concrete, the more imminent, the more compulsive. At each step one had less chance “to call one’s soul one’s own.” To accept the Incarnation was a further step in the same direction. It brings God nearer, or near in a new way. And this, I found, was something I had not wanted. But to recognize the ground for my evasion was of course to recognize both its shame and its futility. I know very well when, but hardly how, the final step was taken. I was driven to Whipsnade one sunny morning. When we set out I did not believe that Jesus Christ is the Son of God, and when we reached the zoo I did. Yet I had not exactly spent the journey in thought. Nor in great emotion. “Emotional” is perhaps the last word we can apply to some of the most important events. It was more like when a man, after long sleep, still lying motionless in bed, becomes aware that he is now awake. And it was, like that moment on top of the bus, ambiguous. Freedom or necessity? Or do they differ at their maximum? (p. 237)