Why is the church bombed?

The Help is set in the early 1960s against the backdrop of the civil rights struggle. The struggle was long and hard, inevitably so, given that it sought to challenge white supremacy in the South and its institutional expression in desegregation and the notorious Jim Crow laws. Birmingham, Alabama was the spiritual and organizational epicenter of the Civil Rights movement, whose leading light was the Reverend Martin Luther King Jr.
One of the civil rights leaders' main meeting places in Birmingham was the 16th Street Baptist Church. On 15th September 1963, a bomb exploded before the Sunday morning service, killing four young African-American girls and injuring many more. Those responsible for the outrage were white supremacists who saw the Civil Rights movement as a threat to the system they had sworn to uphold. They were members of the racist Ku Klux Klan whose Birmingham chapter had a particularly brutal reputation. It wasn't until many years after the bombing that the perpetrators were finally brought to justice.
https://www.history.com/topics/1960s/birmingham-church-bombing