A church has 800 hitpoints.
Each cat you send has 1 offensive hitpoint.
You then need to multiply the offensive and the defensive hitpoints by the offensive (faith, luck, officer, weapon mastery) and defensive (wall, faith) modifiers.
You need to hitdown at least more than 50% of a level to hit it down. What this means is, if you have both modifiers at 100% you would need:
Level 1 church > 401 cats
Level 2 church > 1201 cats
Level 3 church > 2001 cats
Instead of sending 2k cats to destroy a level 3 church you can also send 3 attacks of 401 cats.
Normally it will be behind a wall so normally you'll need +800 cats. And because you don't want to take chances on luck and so on, I'd say you should always send at least 900 cats.