I would also go with common sense. There is no special ammo for shooting into a crowd, and the innocent-target-rich environment is really not much different from a movie theater, a restaurant, a mall, or other venue.

Overpenetration is much less of a realistic concern than simply missing with some of your shots, so pick an effective self-defense caliber and round which you can shoot accurately and practice a lot.

And make sure all your church security team members know CPR and how to operate the fire extinguishers. Probability-wise, you're still much more likely to need that than your guns.