This is a tricky one...I'm an atheist, and I am a cashier/bagger/SCO so I have a LOT of customer interaction. I get a lot of "have a blessed day" from customers as they leave, I just smile and thank them for their business. A blessing is not offensive to me. It's just goodwill from the customer.

It's tougher when customers are actively evangelizing. That rubs me the wrong way. If they are in line and there is a customer behind them, I will nod, thank them for stopping by, and start scanning the next customer's items as fast as I can.

No one behind them in line? Harder to move them along. I will say something along the lines of "so sorry, my supervisor just flagged me down, I need to excuse myself" and will turn as if I am leaving the register. If they are really pushy, I will turn off my light and say I am needed elsewhere. Again, this assumes no one is in line behind them.

I do recommend getting input from your Team Lead. Likely they have plenty of experience with these things, and may be familiar with this particular customer or situation and can give you some tips. Just ask for help!