If God exists, what is the purpose of life?

Theists will always criticize atheists by saying "If there is no god, what is the purpose to life."

But I have come to a revelation. If there is an omniscient, omnipotent, and omnipresent God, a Heaven, and a Hell. . . what is the purpose to life? If God already knows I will go to Hell, than why am I not there? What is the purpose to this life? If there is any purpose, it only proves that God is a sadist. I can't see anything around this.