Talk to your players (especially the priest). Let them know what you expect, see what THEY expect and reach some agreement. With the priest, ask him to create holy days, rituals, habits etc...they don't have to be detailed, just something he can reference. The idea is, with creation comes investment, with investment, interest. And he will want his own creation to make some impact and may actually start pushing other players (at a minimum, he should read deities and demigods for his own deity, paying attention to dogma and vows. You should too so you can know when he does something opposite.
Still with the priest, he needs to behave in accordance to faith. For example, dealing with cultist of some foul god should be much harder for him then dealing with evil assassin. Withdrawing healing (and support) from character who mocks his faith should be rewarded. Doing the same for mocking him (the character, not the player) should not.