1.) Make it mostly useless. (Yeah, PbtA. But in DnD terms it'd be the same as a +1 bonus on dice once, something small like that)
2.) As it's a "reward of faith" that is abstract, you can prop that up with a predefined "punishment" guideline that only the faithful gets, that's a) not too strict, and b ) Slightly more severe than the reward to balance it out. (Something like the OCD rituals in the Anime Darker than Black... Yike that's old nowadays. Still great though. =w=b )
(Remember, failure also advance plots.)
3.) Make it the GM's responsibility to decide the exact requirement for the rewards. That way you can just think on your feet that's situation specific to the plot at hand.