Something that's predestinate has been planned or arranged already — there's no way to change its outcome. Some religious observers believe that life is predestinate, willed by God.
If you believe that people have free will, the ability to make decisions that change the course of their lives, then you don't agree that everything is predestinate. If, however, you think God or fate or nature has already determined what will happen to you, you believe in a predestinate life. This adjective comes from the Old French prédestiner, "ordain of God," from the Latin root praedestinare, "determine beforehand."