Rosa Parks as a symbol
Rosa Parks provided the perfect outlet for Martin Luther King, Jr's political demonstrations, because when she was confronted by injustice (people forceably removing her for a bus for sitting where the white people usually sit during segregation), she simply denied them, stating that they were morally wrong. By peacefully resisting evil, without playing the victim in any way, her peaceful resistance was like an argument against racism. She symbolizes the power of simply obeying one's sense of justice, without being limited by one's fear of the consequences. She is a symbol for having faith and doing what is right.
The pilgrimage allegory
Martin Luther King, Jr's religious philosophy has often been questioned, but this metaphor is such a beautiful, successful metaphor that it's simply infectious. In order to help alleviate the pain of racial instability and mistreatment, King encouraged black people by explaining (through religious metaphor) that they were on a pilgrimage, like the Israelites in the Bible, who were once slaves (as in America), and then, once freed from slavery, had to endure years of desert wilderness, a sacred, painful pilgrimage from slavery to peace.
The boycott itself
The boycott is a symbol. First of all, it's a symbol for the power of social organization, because it would be kind of intimidating if an entire section of the population could organize without other people finding out (especially if it is an oppressed class, because there might be the fear of retribution). But, that's why King was so passionate about nonviolence. By organizing in a hateful way, they would certainly promote endless violence, but by organizing in a way that didn't hurt anyone, the demonstrations could actually work. The boycott symbolizes nonviolence at the societal level, as opposed to the individual practice of nonviolence.
The motif of religious enlightenment
King demonstrates religious enlightenment in many ways. First of all, he sees the broader picture of the Bible itself, looking past normal Christian philosophy at that time toward a new theology centered around social justice and neighborly love. For Christians, it would be hard to argue that he was not a real prophet, because he used the stories of the Bible in a way that was extremely controversial at the time, but which actually changed the nation. It's well known what King himself believed, because he wrote a lot of theology before he died, and it's obvious from those books what his religious beliefs were.
The sacred principle of nonviolence
Nonviolence occupies a holy space in Martin Luther King's imagination. For him, it is the single word that represents true justice, true peace, and true love. If the world is broken, then it is violence that is broken about it, so by accomplishing nonviolence, King believes we can heal our societies. But that's not easy, necessarily, because King defines nonviolence as the desire to make an enemy into one's friend. It involves a voluntary sacrifice, (here's the Christian theology making an appearance) the voluntary sacrifice of one's judgmental feelings, which cause us to become violent and broken. By universal nonviolence and love, he believes we can live at peace.
