Some Christians don't believe the same exact things....surprise surprise! He was still a devout believer in Jesus and God--and a minister. I'm not sure you understand the diversity among Christian sects and the repeated general discussions going on in the theological world.
Here's a prospectus (a preview) for a book series with the writings of MLK Jr that will explain a tiny bit about what he discussed in a paper in theological seminary:
Here's a quote from the prospectus:
"On balance, King shows himself willing to abandon scriptural literalism, remaining confident that this would not undermine the 'profound foundation' of the Christian doctrines."
It's the SCRIPTURAL LITERALISM that he rejected in this assignment, and I have no idea if he continued to think of Jesus or Christianity in the way he wrote about as a theory in an assignment. Sometimes in theology or religious studies, it is an exercise in thought, not a conclusive statement, that a student is urged to strive for. A thinking exercise, not a thing a student must write in stone or commit to for life.
Some people don't consider that the Old Testament stories and their magical events were literally true. Like Jonah in the belly of a whale. That is considered nearly a legend, not a truth, by even the most literal interpreters of the scriptures. Some might argue that the sea turning to blood in the story of Moses was not a sea turned to blood but a sea that was contaminated with a kind of very real red algae. There are non-literal interpretations that allow for science or reason. Pope John Paul II made a public statement to the world saying that the theory of Evolution was most likely true--but that doesn't mean he was a non-Christian or didn't believe in the idea that God created the heavens and the Earth. Do you see what I'm saying? Some people may explain the resurrection of Christ in a medical or SYMBOLIC way rather than as a rising from the dead through magic or miracles. That doesn't mean they don't believe Christ was divine, even. Some Christians don't believe Christ was divine at all--that he was a human whose philosophy, much like Siddhartha Buddha's was absolutely groundbreaking and something to commit to in actions rather than in faith in divinity. They might consider him an inspired person or a prophet instead. Buddha's ideas from over 1000 years prior to Christ were identical to the Golden Rule--almost word-for-word. Some people use Buddha as a divine inspiration but they do think of him having been just a plain guy. I'd say that you can't exclude Christians from Christianity just because they don't agree with you on every single point.
Another thing that the course MLK Jr was taking (as described above): it was systemic theology. Comparing the systems of various religions, in other words. He compares ancient Greek religion to Christianity in that there are similar stories of a child of God (son of God like Hercules) who saves the people; somebody who is immortal or a demi-god who is later taken up to the stars or is made a complete immortal who lives on Mt Olympus, etc, after his or her mortal death. That is a very common comparison made in any study of theology, not some kind of apostasy or heresy in most theological school settings.
Just some thoughts.