A chapel can only be built once between all your villages. It provides faith for your troops. It is also very cheap and built very quickly using barely any provisions.
A church comes with multiple levels, All the levels using a huge amount of provisions. starting at 5k for the first level. It also provides faith for that province.
100% faith on first level, 105 on second. 110 on the third and final level.
If you don't have faith in a province. your troops basically fight like babies. So always make sure you have faith.