Religious people find other religions and non-religious people a threat as it forces them to open up the following question:
"Given that there are alternatives, how can I be sure that I am following the one true way"
They may be pragmatic enough to appreciate that they can't convert Jews, Muslims, Hindus to being Southern Baptists like themselves, as these people are equally 'irrationally fanatic' (where their cultural background, race and faith intermingle in a way that they are wise enough to avoid) - yet atheism and particularly agnosticism present a 'thorn in their side' - there is no culture, there is no race, there is no faith, so in this vacuum they feel compelled to attempt a conversion to their faith (as other religions may wish to convert the non-believer to their respective faiths also). This may come from the notion of 'storing up credit in Heaven by saving souls', although it is really Jesus that is doing the saving in each case, you are supposed to get 'brownie points' for 'leading them onto the one true path to enlightenment'. I don't really think that these Christians are concerned that you will go to hell, as much as what they will get out of their rehabilitation of you - i.e. they will cancel out their sins for all the sinners they 'save'. The fact that Victorian Missionaries spread Western diseases amongst tribal people is just an 'unfortunate side-effect'.
Agnosticism is like a red rag to a bull, as it seems such a weak position to adopt from the point of view of a person of faith (i.e. a zealot).
It would be nice if some of these people actually read the New Testament and adopted its message of Tolerance, but a zealot can always see what they want to see and mentally filter out whatever doesn't fit their personal intolerant interpretation of a "do unto others as you would wish others to do unto you" ethos. Obviously, this problem is not unique to Christians and it should be noted that there are good Tolerant Christians and Muslims, etc. (in the latter case, it is interesting to note that although the Muslim's do not recognize 'Jesus' as the son of 'God', they do accept him as an important Prophet who said wise things...).
Personally, I'm an Atheist (despite not being able to prove that there is no sort of God), because I prefer to live in Reality. I don't accept a world with an intangible Supreme being any more than I believe in Ghosts. Life is complicated enough without creating reassuring fictions - I would prefer to live with the gaps, the unknowns, the discomfort and the prospect of oblivion at the end of my life, than a load of totally unprovable crap. But then, that's me...
