I would stop, and sing ACappella,to keep the momentum going, maybe throw in a chord or note if you can find it. After that, at the earliest opportunity, I would talk to the preacher about the problem. Maybe he doesn't know much about music, and can only sing these songs how he learned them. In which case you'd probably have to learn them in his key. But the time to find this out is BEFORE you perform. Unless he is totally atonal, a little advance communication might solve this. Signed, Dear Abby