You can choose a different cult by resetting your Devotion Points in the Cults' menu. After you reset your points, all the skills you learned become inactive, and you will again receive messages from the emissaries of the three cults. After this, you can choose to worship a different cult by completing one of the three quests. Devotion Points for one cult cannot be used when you unlock branches in another cult's abilities tree, but you can always redistribute points to get new unique abilities in the same cult.