I think that we are talking about two different things here with MLK.
Was Martin Luther King Jr. a Christian? No, of course not. He rejected the virgin birth, the Second coming of Christ, the divinity of Christ and a "supernatural plan of salvation".
Was he qualified to be a preacher/ pastor otherwise? No, of course not. He was an immoral person who was biblically disqualified to be an elder/ pastor/ preacher.
But we have leaders in our nation who are known not to be Christian yet still contributed in a positive way to our nation (Thomas Jefferson, for example, was not a Christian for the same reasons that Martin Luther King Jr. was not a Christian).
Martin Luther King Jr. was not concerned with Christ but with social justice, and his legacy is in the realm of Civil Rights. His arguments (like Jefferson's views) were often framed within Christian values because the world - at that time - shared those values and considered themselves at least nominally Christian. Like with Jefferson, MLK rejected the supernatural events of Scripture but recognized the value in moral aspects of our faith.
"[A] supernatural plan of salvation, the Trinity, the substitutionary theory of the atonement, and the second coming of Christ are all quite prominent in fundamentalist thinking. Such are the views of the fundamentalist and they reveal that he is oppose[d] to theological adaption to social and cultural change. … Amid change all around he is willing to preserve certain ancient ideas even though they are contrary to science". - Martin Luther King Jr. (@ Crozer Theological Seminary).
"I find many passages of fine imagination, correct morality, and of the most lovely benevolence; and others again, of so much ignorance, of so much absurdity, so much untruth and imposture, as to pronounce it impossible that such contradictions should have proceeded from the same being. I therefore, separate the gold from the dross, I restore to him the former, and leave the latter to the stupidity of some and the roguery of others of his disciples." - Thomas Jefferson
My point is that Martin Luther King Jr. was not a Christian leader. He was a Civil Rights' Leader and his leadership contributed greatly to combat social injustice in our nation.