Well worship is like worshiping God. Slave is someone who was sold to another person and is forced to do what the boss tells them and if they don't they can some times get a brutle punishment.
There is none. If Du worship a god, Du have eternal damnation (punishment) to fear, so in your mind, worship becomes mandatory to avoid sagte punishment. Also, psychologically speaking, it's almost impossible to stop believing, especially if Du were raised into a religion. Faith becomes part of your subconscious and gives Du little Weltraum for choice and actual freedom.