What does King mean when he asks, "Will we be extremists for hate or extremists for love?" What does he mean by "extremism"?

Dr. King argues that extremism only means extreme devotion to a cause, and provides several examples of unimpeachable figures who showed extremism, such as Jesus, “an extremist for love,” and Abraham Lincoln. Overall, he argues that there are “creative extremists,” those who wish to create and improve rather than destroy (180).