I was trying to explain to one particular such Christian friend that the problem with religion is that "The fish is the last to know that it lives in water". That is: it seems obvious that "Our religion is the one true religion" only if you hadn't travelled and realised that everyone thinks that, but doesn't agree with you. In a context of a Christian country, a Christian upbringing, and a Christian government, this is not so obvious.
She disagreed, and said very firmly that she does notice such consistent trends. "I would know if I lived in water."
We were in a packed lecture hall. In reply, I said simply:
"Everyone single person in this room is wearing jeans, except me."