Somehow the conversation at the table turned to religion, and at some point the christian in the debate said something like, "I'll pray for you." I politely asked him not to.
A bit more discussion ensued, and he asked why athiests found christianity so offensive. I told him that I would be happy to explain, but only if he was willing to listen with an open mind. He agreed.
I explained that I had no problem whatsoever with anyone believing anything they wanted, so long as that belief stopped at the tip of my nose. I may choose to debate religion with my friends, but in the end I respect their right to believe as they choose, and I expect the same from them.
I went on to explain that the reason I personally found many christians offensive was that they attempted to preach to people who did not believe, and who had no real interest in being preached to. They also tried to bring others into their religious beliefs and practices nonconsensually, with the classic example of this being, "I'll pray for you."
We discussed the fact that when christians do this, it's because they believe that they are doing good and helping the other person. I said that I understood, but that often the person had no real interest in receiving that sort of help and that to try anyway overstepped personal boundaries. I also explained that when christians tried to convert me and preach to me, they were failing to show the same respect for my belief that I did for theirs-- they were in essence saying that they know better, that their beliefs are superior.
"But my religion tells me to preach to others, and help them believe!"
"And that is *precisely* why I find your religion offensive."
I'm glossing over a lot of what was said-- it was actually a three-person conversation not two, but the most interesting parts (to me) were my discussion with the christian. In the end, he said he had a much better understanding of why nonbelievers or people of other faiths would be bothered.
The whole conversation was civil and respectful from end to end, and may have actually resulted in some small degree of enlightenment. I'm happy with it.
n.b. There are generalizations in here. Please interepret them as "many christians do X", not, "all christians do X."