“The lessons which I was taught in Sunday School were quite in the fundamentalist line. None of my teachers ever doubted the infallibility of the Scriptures. Most of them were unlettered and had never heard of Biblical criticism. Naturally I accepted the teachings as they were being given to me. I never felt any need to doubt them, at least at that time I didn’t. I guess I accepted Biblical studies uncritically until I was about twelve years old.7 But this uncritical attitude could not last long, for it was contrary to the very nature of my being. I had always been the questioning and precocious type. At the age of 13 I shocked my Sunday School class by denying the bodily resurrection of Jesus. From the age of thirteen on doubts began to spring forth unrelentingly. At the age of fifteen I entered college and more and more could I see a gap between what I had learned in Sunday School and what I was learning in college. This conflict continued until I studied a course in Bible in which I came to see that behind the legends and myths of the Book were many profound truths which one could not escape...
My days in college were very exciting ones. As stated above, my college training, especially the first two years, brought many doubts into my mind. It was at this period that the shackles of fundamentalism were removed from my body. This is why, when I came to Crozer, I could accept the liberal interpretation with relative ease...
At the age of 19 I finished college and was ready to enter the seminary. On coming to the seminary I found it quite easy to fall in line with the liberal tradition there found, mainly because I had been prepared for it before coming.”
“An Autobiography of Religious Development”
http://mlk-kpp01.stanford.edu/index.php/encyclopedia/documentsentry/doc_501122_00
It does seem that when he was twenty, King wrote essays as though he held a materialist view of Christ.
Bottom line is, if he didn’t believe in the Resurrection he wasn’t Christian. But religious beliefs often change after age twenty.