Maybe there isn't a clear cut line, but the Bible is straightfoward. Jesus is the answer, everybody else is for themselves after we die.
I guess I just don't see how somebody can be Christian, Muslim, Jewish, etc...and still believe everybody else has an equal shot of getting into heaven. I don't say this to offend, but it seems like wishy-washy faith to me. There is something qualifying one faith over another and the choice is personal.
Most are a specific faith for a reason. If one believes in the Christian notion of God, simply because they are raised that way but don't necessarily believe in everything (ie, the buffet approach of picking and choosing) then they are saying their basic faith and sprituality is due to the sheer accident of birth, but will believe what they want ultimately. If they say they see the most value in Christianity because it makes the most sense and they like the foundational beliefs (salvation, grace, etc...) then it is hard to say everybody else has an equal shot at heaven, given those beliefs.
I just think if somebody says they are Christian, believe in Jesus, and read the Bible, they should have no qualms about looking at Muslims beliefs, or atheists, or whoever else and saying "Respectfully, you're wrong."