One of the basic human rights, though not expressly mentioned in any Constitution, is the right to be wrong. So, who am I to tell someone else that he is wrong. Heck, I know that I am wrong. And guess what - it does not bother me.
I am not an absolute being, nor am I a Buddha, someone who has it all figured out. I don't have all the answers.
That said, if they started a religious dispute, I might tell them what I think. Or if they asked me, I would tell them. But I would never start such a dispute, nor would I try to incite it.
Even if they start, I don't always contradict them. Sometimes just keeping quiet is the best reaction (neither agreeing nor disagreeing).
Once I was in the local bookstore when a man came and started talking religion in ways that I absolutely disagreed with. But he was so obviously convinced about what he was saying that I just stayed quiet.
After he left, the bookseller said she knew I did not agree with a single word of what the man said and was surprised I did not tell him (I always tell her - but she always starts and she is a friend and I know her, know deep within she is curious about what I believe, and she can take it just fine).
I told her I was not willing to take the man's beliefs away unless I was ready to stay with him and to fill the void I would create in him.
She then called me a very kind man.