The church I went to when I was younger was very small, but they still managed to haul in at least $1000 in offerings every week. Do your research and then target the church that rakes in the most money, they have the amounts in their bulletins.
Just do a little surveillance to find out where they put the money after they take it up. Then breaking in shouldn't be much of a problem, because churches usually have little security. It usually goes in a safe though so bring the usual safecracking tools, if your surveillance suggests that they do.
If you're any good at sleight of hand you could be able to take money right out of the offering tray when they're passing it around. Sit in the back, so nobody behind you can see you. Really depends on the church though.
People also seem to feel pretty safe about their possessions in church. SWIM used to pocket valuables all the time from unsuspecting church goers.
Robbing banks is a thing of the past, move on to churches folks.
(pretty much copied from my post in a thread on &Z. Added some stuff though.)