One way that we can tell Dr. King is a Christian is to examine his writing.
Dr. King makes repeated appeals to Christianity in his "Letter from Birmingham Jail." In the letter's opening, he states that he has "the honor of serving as president of the Southern Christian Leadership Conference," one whose affiliate is "the Alabama Christian Movement for Human Rights." Dr. King does not shy away from his association with Christianity. The fact that he leads these associations displays his Christian faith.
Dr. King also identifies himself as a Christian on a spiritual level. His faith is the reason he says he is in a Birmingham jail. When he writes that he is in Birmingham because of "injustice," it does not take long to see that his spiritual identification is the reason:
Just as the eighth-century prophets left their little villages and carried their "thus saith the Lord" far beyond the boundaries of their hometowns; and just as the Apostle Paul left his little village of Tarsus and carried the gospel of Jesus Christ to practically every hamlet and city of the Greco-Roman world, I too am compelled to carry the gospel of freedom beyond my particular hometown. Like Paul, I must constantly respond to the Macedonian call for aid.
Dr. King identifies himself with figures from the Christian faith. His reference to Christ and the apostle Paul shows that he validates and embodies the teachings of Christianity.
Dr. King looked at the issue of Civil Rights as a spiritual call to action. He believed that his role as a Christian compelled him to act. We can tell that he is a Christian because he is open about his faith. What most saw as a political issue, he sees as a spiritual one. As a result, Dr. King was able to broaden the Civil Rights debate to millions of other people who saw themselves as "Christian."