I figured it might be a good idea, even this late in the thread, to response to the OP first.
QUOTE(Irish @ May 29 2007, 09:24 AM) [snapback]1699031[/snapback]
If Christianity was a pyramid scheme I could understand the mistrust associated with Christians here on the forums.
If they were trying to get you to join their church to bolster their numbers and increase the financial gains of the church, but I don’t see that happening here.
But that usually is the case with much of Christianity, and more than a few churches treat the convert business as exactly that- a business. The Mormons are somewhat notorious for their business-like approach to getting converts, although Scientology, a non Christian faith, is still the most naked in pursuit of a potential convert's money.
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Christians have nothing to gain personally from any conversion except perhaps a friend.
That's hardly true. They gain the self-satisfaction of having brought a convert into the fold, which also can reflect on them fulfilling their duties to God (if I am correct, part of the beliefs of much of Evangelical Christianity is that you are obligated to spread it). If they are in an area where their presence is weak, then they can build a community from it.
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Then why do they want to share their faith with others constantly?
Many on these forums see the word faith and immediately set out to debunk outright as it is were a fire that needs to be extinguished immediately.
Let’s look at it this way. If you found that the cure for cancer was as simple as sugar and water taken at precise times of the day. Would you share this news with joy and zeal? If you are a normal caring human off course you would, with as many people as you can. Even if you did not understand how or why it works you would still share the method. You would relate your experience with the method as well as others testimonies as well.
A more appropriate example would be if you believed that you had the cure for cancer, and wanted to share it with others - only, they already have their own cures for cancer, the evidence for your cure being more effective than theirs is dubious, and many doubt that the cure is even a cure at all.
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Well Christians believe with all their heart, mind and soul that they have found the one and only cure for death, the secret to immortality and the fountain of youth all rolled up in a single Messiah They would be insensitive to not share with others. Even if only one other person benefits from it and most people reject it.
They are willing to take the ridicule and humiliation that goes along with sharing something so simple. Yet they willingly allow themselves to be insulted and degraded by those that choose not to believe them.
They take it now -because they have to. That wasn't typical with much of Christianity's history; missionaries, particularly catholic missionaries in the form of Jesuits, followed in the wake of European imperialism, and frequently took advantage of their superior power position to put down questioning, suppress the local religions (the Inquisition was active in the New World as well as the Old), and force their religion onto the conquered.
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If their motivation was purely selfish smug then all they need do is keep their mouths shut and we shall all find the truth when we leave this world.
Not if you believe that the unrighteous are preventing the Second Coming, or are afraid that the influence of the infidels is going to cause your children to go astray. Or that your position, privileged within a Christian society, will erode when that Christian glue and hierarchy is replaced with something more secular. Or if you believe that it is your obligation to save unbelievers from Hell, regardless of the cost. Or if they're simply using the Christian banner to further their own ends.
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So just what is it you think motivates a Christian in telling others of their faith and joy and how they achieved it? And why does it annoy and anger a lot of people?
It's annoying because it reeks of self-righteousness, and is tainted by the fact that Christianity, although it has served all sides, has frequently been the banner of xenophobia, and its current equivalent, homophobia (don't forget, too, that many of the American Christian Right leaders that were around before the 1960s were racist, as well). If a muslim rubbed it in your face that he believes he is saved and you are damned, then it would get quite annoying.
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It’s a simple “believe it or not”, choice yet the reaction from both sides are often hostile and rude, and why does that happen?
It's not that simple. Missionaries frequently will use emotional weakness in the receptees to their advantage in getting a conversion.
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One objection I hear a lot, more from the non believers is the threat of “Hell” Why would hell be a threat to someone who does not believe in it? If someone told me I was going to Bonga Bonga if I don’t believe in BoB. I would be more concerned with whom the heck is BoB?

Bonga Bonga would not even be on the radar at that point.
And to most Christians hell is just the eternal separation between man and his Creator and non believers are already riding that club car anyway.
Because it's an appalling concept, akin to telling someone that "I like you, but I'm fine with the fact that you'll be tortured unless you act like I do". Seriously, the fact that this doesn't bother more christians than it does is significant in of itself.