Carmen mentioned the character of Emeth, from C.S. Lewis' The Last Battle (cf. especially chapter 15), who served a false god, but in such a way that Aslan (the Christ-figure) nevertheless recognized him as a true follower of his. Of course, this is fiction, merely a thought experiment like the rest, but it might be helpful to supplement it with one of Lewis' more explicit statements on the subject. This is from Mere Christianity, and I agree with it completely:
Is it not frightfully unfair that this new life should be confined to people who have heard of Christ and been able to believe in Him? But the truth is God has not told us what His arrangements about the other people are. We do know that no man can be saved except through Christ; we do not know that only those who know Him can be saved by Him. But in the meantime, if you are worried about the people outside, the most unreasonable thing you can do is remain outside yourself. Christians are Christ's body, the organism through which He works. Every addition to that body enables Him to do more. If you want to help those outside you must add your little cell to the body of Christ who alone can help them. Cutting off a man's fingers would be an odd way of getting him to do more. (pg. 64)
UPDATE: This post is part of a continuing conversation.