The entire movie is an arguement for Theism, specifically Christianity. Frank is God. Donnie is Jesus. Frank shows Donnie the things that will happen if he doesn't die, but Donnie is afriad to die because he isn't sure if God is real. Frank shows Donnie how everything in life is predetermined (the wormholes coming out of characters' chests). Donnie believes this predetermination is God and decides to die.