I was having a conversation about what it means to be a Christian, and a friend asked me, "Do you never wonder if you've got it wrong?" Here is my response:

I do wonder if I've got it wrong about a lot of things. The difference is regarding how clear the Bible is. I'll give you two examples:

[First example is fairly reasonable; he doesn't believe a Left Behind-style Rapture will occur, but admits he could be wrong since the Bible isn't totally clear. Then comes example 2...]

Homosexuality- Liberals who call themselves Christians believe that homosexuality is not sinful. Personally? I don't think so. But, in contrast to eschatology, the Biblical case is much different here. Not only are there over a dozen verses/passages in the Bible that explicitly state that homosexuality is sinful, but there are zero verses in the Bible that say anything leading to the conclusion that homosexuality is morally acceptable. So we have over a dozen verses on one side and zero verses on the other side.

This is why the issue of homosexuality is not an issue of interpretation. If it were, I would not question the salvation of those who disagree with me (and I affirm the salvation of many who disagree with me on Eschatology). No, the underlying issue of homosexuality is whether or not you accept the authority of Scripture, or if you would presume to say that God's revealed word is wrong in its dictates.

This is why those who claim Christianity but affirm homosexuality are incredibly arrogant. They acknowledge that God Himself has spoken to us, but they reject His Word and replace it with the teachings of evil men who are in rebellion against God. It is also the reason that they are not Christians. The first, most basic characteristic of a Christian is that they affirm the authority of the Bible. Those who affirm homosexuality have rejected the authority of the Bible, so they are, by definition, not Christians.