They also lie about their qualifications. Like Phelix claiming to be a biblical authority, names like "SuperlativeCritic" and other such examples. I'm guessing that at least part of it comes from insecurity. It tends to be the ones with the greatest insecurities that are the loudest. When it comes to religion the more that you know about it the more obviously insane it is. So you have one of two options, either look at it logically and realize that it's all ridiculous. Or lie to yourself until the feeling of cognitive dissonance goes away. Then you have to keep lying in order to maintain your self-imposed delusion until it becomes natural and you don't even realize that you're doing it anymore.
One word: Majesty.

I think we should be careful to remember that the Christians who come here, especially the ones that end up in the Emergency Room, are a self-selected sample, and not a legitimate basis for generalization about Christians or theists in general ("theist liars"). We would not, for example, likely see someone like
Fred Clark coming to a Forum like this, and him ending up in the ER? Inconceivable.
I think a key explanatory factor is that there are multiple levels or senses of "belief." In particular, there's the belief that something is real ("I believe in evolution") and the belief that something is right, good, proper, valuable, etc. ("I believe in single payer health care"). Theists (especially fundamentalist types) tend to talk as if their belief is of the former type, when it is actually of the second type, or what Dan Dennett calls "belief in belief."
The first level is the level of expectation or anticipation of what reality is like. Few believers actually expect reality to behave as if they have a Personal Relationship
tm with an omnimax superbeing. They do not expect angels to actually appear and get their car out of a rut in the mud, even if they're on their way to preach the most important sermon of their lives. They don't forgo insurance policies, college funds and retirement plans because they're confident that Yahweh is watching out for them. They already know, in advance, that reality will behave as if it were abiding by naturalistic principles and that the dice of probability are not rigged in their favor.
We can know this because all the theological justifications for
anything reality might do are already in place.
[1] "God works in mysterious ways." "Two boats and a helicopter" Person improbably survives a disaster: "Halleluiah! It's a miracle!"
[2] Person dies in the disaster: "God wanted to take them home." "Yes, No, and Wait." For the most part, or at least with regard to anything that actually matters, their beliefs are hermetically sealed from any kind of reality-test. When the belief is "My
real Daddy is an omnimax superbeing who loves me
soooo much he created a hundred billion galaxies just so I could be here!," creating an effective quarantine that keeps it from being contradicted by a godless Universe--or to be precise, the believer's own generally-accurate
mental model of a godless Universe which lets them anticipate the behavior of reality well enough to get around--takes some serious doing.
The second, "belief in belief" level of theistic belief is about believing in the virtue and/or usefulness of
having the belief itself. For many (probably most, but I don't have survey data) Christians, belief in Yahweh is not an issue of reality-anticipation ("truth"), but an issue of
morality. Belief in Yahweh is either the beginning and the end of all moral goodness
[3] or at least the passkey that makes genuine moral goodness possible. This is the underlying premise of the belief that only Christians can go to Heaven. Belief in something else (or lack of belief) is not merely mistaken, it is
morally wrong, so wrong as to be deserving of the ultimate punishment. Once you take something like that on board, then pretty much
anything one might do is better than questioning, much less rejecting belief. If you lie to some atheists on an internet forum, you can limp away to the nearest altar-call and plead for Jesus' forgiveness. If you start to lose your faith (or lose an argument in the attempt to promote your faith) while still believing that having faith=morality then not only is your moral goodness in peril, so is the possibility that Jesus will offer you forgiveness and a clean slate again.
Then there's all the real, practical things the believer stands to lose if their faith falls. Their believing friends, their church community and all the advantages it offers, their spouse and children, their membership in the respectable norm (if they're Americans), all the time and money (sunk costs) they've already invested in their faith, and sometimes their livelihood or their dreams. The cost of giving up belief, or even acknowledging that giving up belief is a thinkable option, is
high.
"Why Won't God Heal Amputees" is basically another way of saying, "Why Can't You Anticipate That Reality Will Behave As If It Contains an Omnimax God?" So, any believer considering joining this Forum knows going in that battering rams and shaped-charges and gigawatt-class lasers will be brought to bear against the hermetic containment structure that quarantines their precious beliefs from reality. The very existence of the structure and the meticulousness with which it is constructed and and must be defended in everyday life
[4] is a testament to its importance for the believer. So, what kind of believer is most likely to be willing, even eager to put that structure at risk by charging into a place like this? It says right there on the label that faith will be held to the standard of a reality-test as concrete as a regrown limb. This is a test that faith simply cannot meet, otherwise the containment structure would not exist.
Apart from a tiny handful of exceptions that prove the rule (Old Church Guy, UnkleE, Think and Answer
[5]), the more intelligent, rational, and honest Christians aren't going to come here. "WWGHA? That's a silly question, because faith isn't about anticipating the behavior of reality, that's what science is for. Faith is about things like meaning and community and moral values and finding inspiration and strength to get through life, or maybe even do something great, like Martin Luther King." Then they move on. This is "more intelligent, rational, and honest" because such believers at least understand that they've constructed Non-Overlapping Magisteria for science and faith. When they set up their hermetic containment structure, they are self-aware and honest enough to accept that they're doing it.
It seems to me that the type of believer most likely to come here, or end up in the Failbag (when that was working), are the ones who respond to the "WWGHA?" challenge from their lizard brain. It's not so much a question about truth/reality as it is an insult to their ego, which is wrapped up in their belief-in-belief. *Thumps chest* "
My faith is so strong, so sure, than I'm gonna come in there and take alla ya on, singly or in groups! Bring it on, pimp juice!" Then, having charged recklessly in and finding themselves hopelessly outclassed (not just by the sharp thinkers here, but by the fact that reality is heavily biased in our favor), this type of believer is forced into a spin control/ego-defense mode. And since consistency with reality was not something they valued in the first place,
[6] it's not too hard to throw under the bus.