What do readers know about the letter he received? How do we know this?

The newspaper allegations he's responding to?

Dr. King explains that he has read the recent statement published by clergymen in a Birmingham newspaper, describing Dr. King’s recent activities in the city as “unwise and untimely.” Though he does not usually respond to criticisms – he receives far too many for that to be practical – he believes these men are “of genuine good will” and hence do their criticisms deserve an answer (169).

He first acknowledges the criticism that he is one of many “outsiders coming in” to cause trouble (their words). He explains his purpose: he is the president of the Southern Christian Leadership Conference (SCLC), based in Atlanta but operating throughout the South. He describes the extent of the organization’s reach, and then explains that one of its affiliates in Birmingham had invited the SCLC to “engage in a nonviolent direct-action program” when racial issues grew difficult there. The SCLC answered the call, and hence does Dr. King insist that “I, along with several members of my staff, am here because I was invited here” (170).