First, it is inappropriate to make assumptions about a customer's religion and to share those assumptions verbally while on the clock. Not being particularly Christian myself, I don't understand the significance of the tilak in Christianity, but I can guess there are many crossovers in religions. You said you weren't particularly offended, but it still is inappropriate.

Second, it is also inappropriate to "punish" a customer for not sharing the same beliefs as you. The cashier in this case did so in a subtle way, but it doesn't change the behavior.

There is a also a third part to your complaint that you may not even realize: they are charging more for a veggie sandwich than they are for one with meat. I realize this wasn't the focus of your complaint, but I think still think it's wrong.

It doesn't look like your complaint was addressed to anyone in particular, so hopefully it will get to the right place.