Just because someone is an agnostic or even an atheist doesn't mean that they haven't spent time studying the bible or even in church regularly. I would consider myself an agnostic and I grew up going to catholic school and as an adult I even took a year long bible study course, just so I could know more about the bible and Christ's teaching. I've gone to evangelical churches, methodist churches and even unitarian. Point is, I know a good deal about the bible and Christ's teachings, likely more than most professed christians. I would never claim to know what Jesus said better than anyone (believer or not), but I as an agnostic know enough about christianity to have a meaningful discussion with any believer, and I have done this many times. I think there is a HUGE difference in 1. having a discussion for the sake of learning more about the topic at hand and the person you are having the discussion with and 2. trying to sway someone to your side. The first can be enjoyable, the second is a fools errand.