1. If God does not exist, then there is no cosmic purpose. (Premise)
2. If there is no cosmic purpose, then human purpose is illusory. (Premise)
3. Human purpose is not illusory. (Premise)
4. Therefore, God exists. (From 1 - 3)
Arguments such as these will appeal to those who believe that human purpose is not something conventional or relative. Someone like Sartre would maintain that we create our own purpose, but ultimately such purpose is illusory. This argument is called "existentialist" because it turns Sartre's view on its head and embraces a position more akin to Kierkegaard's. I don't maintain this is a proof or demonstration of God's existence, but I do think it constitutes one of many rationally acceptable reasons to believe in cosmic purpose and, ultimately, God, the giver of purpose to the cosmos.