From the beginning to the point of no return, that's our Act I. In Go Tell It on the Mountain that's from the minute that John (our main character) wakes up on his fourteenth birthday. He heads into the city to catch a celebratory movie, and back home to find his brother, Roy, laid up because he got stabbed. A fight ensues between Gabriel (the father and paterfamilias) and pretty much everyone else in the family. This sets up all the family dynamics that determine the rest of the novel.
The first act follows John into the church that evening, where he starts to feel like he might be overcome by the Power of God. It ends with him wavering, staring into Gabriel's eyes and on the edge of being overcome by the Spirit.
Act II takes us from this point up to the scene where John is lying on the floor, overcome. He feels the Power of God but doesn't think he can make it through his pretty gnarly vision: he sees graves and despair.
Zigzagging through all of this spiritual ecstasy are the memories of John's family members. We find out that Gabriel is actually John's stepfather, which is why he treats John so badly. He's an evil stepfather.
Phew, we made it. John gets through his spiritual journey with the help of the prayers around him. He wakes up on the floor of the church as a grown Christian man, ready to confront Gabriel, and everybody heads home. That's a wrap.