the functions of defender of faith are somewhere in the beta information 
anyway it gives a bit less RR added and lower RR cap for war exhaustion.
you lose like 25% tech speed for the duration, with the current rebel rules the reduction in warexhaustion RR is really meangingless compared to losing 25 tech speed
(not numbers may not be accurate just fetching it from memory)
also defender of faith gives free CBs on other religions or something.
anyway it gives a bit less RR added and lower RR cap for war exhaustion.
you lose like 25% tech speed for the duration, with the current rebel rules the reduction in warexhaustion RR is really meangingless compared to losing 25 tech speed
(not numbers may not be accurate just fetching it from memory)
also defender of faith gives free CBs on other religions or something.