In the opening paragraphs of "Letter from Birmingham City Jail," Martin Luther King comes across as a strong leader, religious, and idealistic.
As for his stature as a leader, King includes comments in his first paragraphs that build his credibility (ethos) by showing he is an important person. He states, for example, that
If I sought to answer all the criticisms that cross my desk, my secretaries would have little time for anything other than such correspondence.
This establishes that he is a person of stature who has a staff of people under him to deal with the "small" stuff while he steers the ship. He may be in jail, but he is still important. He also states that he is
president of the Southern Christian Leadership Conference, an organization operating in every southern state, with headquarters in Atlanta, Georgia. We have some eighty five affiliated organizations across the South, and one of them is the Alabama Christian Movement for Human Rights.
The above explains why he crossed state lines to enter Alabama, but also, more importantly, establishes his credibility and leadership. King states all of this on purpose to assert his authority: he is implying he is the equal of the white people to whom he writes.
King also establishes his religious credentials and beliefs early on. He compares himself to no less than the apostle Paul, spreading the gospel. This is an audacious claim and one that aligns the civil rights movement with Christianity itself. King says that he, like the
Apostle Paul left his village of Tarsus and carried the gospel of Jesus Christ to the far corners of the Greco Roman world...
It will be hard for other clergy to attack a person following in Paul's footsteps.
Finally, King's idealism shines out as he speaks of working for justice. He states that all people are connected and that injustice for one is injustice for all. He is not in Alabama out of self interest or for personal gain, but for the greater good of all of society. He states,
Injustice anywhere is a threat to justice everywhere. We are caught in an inescapable network of mutuality, tied in a single garment of destiny.
In these opening paragraphs, King is establishing his own ethos as a person of good character and as a leader who is driven by faith and ideals. These are good rhetorical (persuasive) moves because they put him on the high ground as a person of authority. It is only after King establishes his own character that he turns to logical arguments and what his movement hopes to achieve.