Based on the beliefs of Marnus. That there is no god or after life. One of the most important points is to live your life in such a way that you make the most of this life and not expect to do so in the "next". Another important aspect of Marnism is to be an original person and not a sheep as most religious/indoctrinated people are. It stands in the way of mediocrity by encouraging the follower to be their own unique self(Just like everybody else).
Tyler converted to Marnism when he laughed at the Christian play.
Joe converted to Marnism when he finally saw reason.
Marnism + Originality = Evolution.