There is a real issue here. What you believe should not be inflicted on other people. it's socially unacceptable, and the fact that you believe in a religion that tells you that you have to witness to other people does not make it easier.

Howver, I repeat, talking about a god that other people don't believe exists is offensive because they are forced to listen to you when they think you are talking nonsense. You may not be talking nonsense, but to force people to listen to something that they don't want to listen to is rude. You should be sensitve enough to switch topics immediately.