There's no need to rationalize carrying a firearm any more than you should need to rationalize wearing a seat-belt, or a helmet, or having life insurance. If you knew precisely why you were carrying a firearm, you would avoid that situation rather than put yourself in it...
Otherwise, I completely agree with jvandivere above. Get permission. Ask that it be in writing - in fact, I would type up your own permission letter, and bring a notary to the pastor to have him sign it with a witness present, thus eliminating any potential for confusion.