I AVOID talking about Christianity.
I also ignore people who bring it up; I mumble, "Um-hum, that's nice," and LEAVE.

It quickly trains people to avoid the subject.

I don't bring it up unless necessary. Why invite trouble?

I don’t ever raise the subject first. It is not a good conversation starter! However, I don’t shy away from it if it comes up. I always make clear my views politely but firmly, and try not to get into heated debates. If the other party becomes aggressive as sometimes happens when some are defending their beliefs, I just say I think we will have to agree to differ and close the argument down.