
Max M. answered 05/06/20
Harvard Literature major with 20 years of coaching writers
The first several paragraphs of "Letter from a Birmingham Jail" answer the outsider charge. The first is practical, outlined in the second paragraph, where he describes the organizations he has relationships with that invited him to Birmingham. As he summarizes it, "I am here because I have organizational ties here."
The next few paragraphs describe his religious and civic duty to see justice done. He brings up notable figures from the bible who traveled to spread their message and to fight injustice, and also reminds the clergymen that his identity as an American is more important than what state he's from. He may be from Georgia and now in Alabama, but they're both in the USA, and that's more important. One key phrase from this section is, "Injustice anywhere is a threat to justice everywhere." Later in that paragraph, he addresses the outsider charge more directly.
Which is more powerful of course depends on you. There's a straightforward, practical answer that you may find very hard to argue with, even if it's not the most interesting part of the letter. The more moral, ethical argument may inspire you, even if it requires the listener to share his definition of ideas like "justice." It's a balancing act, and there's probably a reason why he gives two responses instead of just one. Which *do* you find more powerful?