If non-belief is a belief and that's what you're stating, then how are you being any more combative or antagonistic than someone who believes that Jesus was unquestionably the son of God? Why can't you be open to the idea that Christianity started as a small cult not so different from the cults of today, and then was adopted by the Roman Empire in an attempt to stop the infighting caused by polytheism and slow their decline, and didn't just succeed because it's inherently 'true'? If that's what I believe, then why are you judging me for it, suggesting I'm 'flaming' or 'trolling'? It was an answer to the question proposed by this thread-- it's on-topic, and I think it's a pretty valid theory.
It's not really a question of Christians being willfully antagonistic so much as the fact that all belief systems are NOT as compatible as 'they're all part of the same truth', especially now that people are free to believe whatever it is they want to believe and a growing number are recognizing Christianity as no more valid than atheism, except in that it is held as a (waning) component of our nationalistic identity.
For some people there is no truth, or truth can only be described in terms of what it is not-- in which case, it is not that Jesus was the son of God, it is not that there even is a God, and not that you really need to worry about doing/being anything or behaving in any particular way... there is no 'movement' from this place to another place, there is no judgment, no accountability, no 'plan', no angels, no demons, no heaven, no hell, no Adam, no Eve, no apple, no sin... if that is my belief, then can you accept that without judging me or getting defensive?
It seems like a lot of the more 'open-minded' Christians actually seem to hold the belief that 'you actually ARE Christian, you just don't know it yet, and we really hope for your own sake that you confront this fact soon and fall in line with what we believe'...the whole 'his truth is written on your heart' thing, for example. I think that that is what so many non-Christians find so offensive-- the idea that everyone and everything falls in line with a specific set of beliefs which also seem to depend heavily on the idea of morality being natural, spiritual accountability, conformity and exclusivity.
The popular image of a Christian is self-righteous, judgmental, conservative and hateful... and while I don't think that this is altogether accurate in all cases, I do think that there are degrees of that in almost every Christian I've ever met.