what made the scene where the church burnt down so importante? what was the underlying message of this scene?

As Ponyboy charges into the school, he "remembered wondering what it was like in a burning ember, and I thought: Now I know, it's a red hell." The burning of the church has of course been foreshadowed (when the boys lay in the vacant lot watching the stars and Ponyboy looked at Johnny's cigarette end, wondering what it was like inside a burning ember.)

Johnny's eyes, a running theme throughout the novel, change dramatically in this chapter as he acts heroically. Ponyboy notes that "that was the only time I can think of when I saw him without that defeated, suspicious look in his eyes." Even at the beginning of the chapter, "his big black eyes grew bigger than ever" at the thought of going to jail, since he was afraid of the police; now, he is confident and acting like a hero, and the change is reflected in his eyes. Once again, Hinton uses concrete physical details and splashes of corporeality to suggest the inner workings of her characters; it's a kind of skin-level expressionism at play, reflecting as it does an adolescent's dawning awareness of the world around him - a world in which eyes are windows to the soul, haircuts and dress styles determine a boy's place in society, and entire relationships are refracted through lingo.