Simulation: 775 Catapults, level 20 wall, no defensive units, varying luck, level 3 church.
Successes: 7% luck or higher to take level 3 church to level 2.
Simulation: 2,500 Catapults, level 20 wall, no defensive units, varying luck, level 3 church.
Success: 4% luck or higher to take level 3 church to level 1.
Simulation: 3,800 Catapults, level 20 wall, no defensive units, varying luck, level 3 church.
Success: 8% luck or higher to take level 3 church to level 0.
Simulation: 750 Catapults, level 20 wall, no defensive units, varying luck, level 2 church.
Success: 9% luck or higher to take level 2 church to level 1.
Simulation: 2,500 Catapults, level 20 wall, no defensive units, varying luck, level 2 church.
Success: 7% luck or higher to take level 2 church to level 0.
Simulation: 750 Catapults, level 20 wall, no defensive units, varying luck, level 1 church.
Success: -2% luck or higher to take level 1 church to level 0.
I'll update this will screenshots, lower cat numbers, and lower luck percentages as I get around to it. xP
Edit: Keep in mind, these are just the values I've found. These are by no means the absolute lowest.