This work is a modern masterpiece and what makes it extra poignant is that it took place within the lifetime of many of us. Let me offer a few rhetorical strategies that Martin Luther King uses.
First, he uses the subjunctive mood or uses the idea of potentiality. This gives an opened ended feel to the whole paragraph. If you notice, he uses the word "perhaps" two times right in the beginning. This strategy allow him to say that there has been gains, but that these gains are not enough. Perhaps he has been too hopeful. In particular, he points out that white moderates are not doing enough.
Second, he also gives a few concrete examples of white who have stood by him and the movement to create equality. In rhetorical terms he has given an "exemplum," or an example. In other words, he is giving a model for people to follow. He writes:
"Some -such as Ralph McGill, Lillian Smith, Harry Golden, James McBride Dabbs, Ann Braden and Sarah Patton Boyle--have written about our struggle in eloquent and prophetic terms. Others have marched with us down nameless streets of the South. They have languished in filthy, roach infested jails, suffering the abuse and brutality of policemen who view them as 'dirty nigger-lovers.'"
Third, he specifically points out that he has been disappointed with the white leadership in the church. By doing this, he shows the hypocrisy of the church, because if the church is called to love the way God loves, then they should be in the forefront of fighting injustice.