He wasn't the sole reason behind the movement - that's for sure. Not a historian (lol) but I'm writing an essay on his contribution now. I think he simply instilled the character needed into the African-Americans to successfully force the federal governments hand through his powerful speeches. Before 1963, he was involved but not a leader. His nonviolent methods were also reversed pretty swiftly during the late 60s with the Black Panthas etc. Nevertheless, he is absolutely crucial in the foundations of the whole movement. He definitely accelerated it.