I felt like that once. If loss of faith can happen to me, it can happen to anyone, Alan.
I've pointed this out to Alan in the past. Of course, he doesn't accept it, given his penchant for speaking in such absolutist and dogmatic terms as he always does (i.e. "I absolutely know," "Nothing could ever change my mind on this," claiming to know rather than to believe, such-and-such a thing is impossible, etc.)*.
The saddening thing is that it's always the people who are this heavily invested in their worldview who go utterly to pieces when it falls apart, as sometimes happens. It's a bit like the boxing adage of "The bigger they come, the harder they fall." The more utterly and flatly dogmatic someone is about their adopted worldview, as Alan is about his, the greater the devastation when it evaporates. More fluid-minded people who sit looser to their attachments - it could be a political ideology as well as a religious one - feel pain and anxiety and upset, certainly, but weather the storm, roll with the punches and come through, many feeling better for it afterwards. I know this is true of you, Rhi, as you've said as much more than once in the past. (My own entirely subjective opinion based on accounts of people to whom it has happened is that a great many of them are better human beings for it, too, not only in having cast off some ugly and obnoxious views in some cases but in being more flexible, fluid thinkers, larger of mind and sensitive to suffering). It's those one-true-wayers for whom their ideology is absolutely everything in their life, those who can't admit any possibility of error or alteration, people who don't or maybe can't do subtlety and nuance and fine degrees of difference - rigid thinkers or rather, I should say, believers whose beliefs are as fixed and as limited and limiting as train tracks - who tend go to pieces when it does. When you put all your ideological eggs in just the one basket, if those eggs end up smashed you're screwed, emotionally, and that's a whole lot of mess to clean up.
There are reasons why some people are this way - some people are temperamentally more uncomfortable with change, uncertainty and nuance than others, and retreat into dogmatic and absolutist ideologies (sometimes one after another after another) because they appear to offer a (quite illusory) bulwark of changelessness, black and white certainty and either/or clarity in a world built upon constant change, ambiguity, nuance and murky greys. Certain personality types seem naturally to gravitate to religions and political beliefs (sometimes a combination of the two) who purport to have all the answers to all the questions ready-made and laid out for the taking. It's always quicker and easier to buy a suit off the peg than it is to start cutting cloth and making your own. A microwave meal for one or a trip to Burger King is faster and simpler than cooking a complex meal from scratch. But however understandable that may be, we need to remember that that desire is a failing and a fault, not an admirable trait or a positive virtue to be applauded, and however superficially engaging such people may sometimes be, they are in essence extremists who are a hair's breadth away all the time from fanaticism. What's often known as junk food is so popular because it's big on all the things - fat; salt; sugar - that give us an immediate pleasurable pay-off but are ultimately bad for us; rigid, absolutist dogmas are the junk food of the mind. It's quite possible to have the occasional burger or pizza as a treat, enjoy it and to live well and healthily, but there seems to be far less of a brake on people who feel that they're in possession of the ultimate truth of the universe and that the search is now over. The universe is full of mysteries but this isn't one of them; we don't need to guess - we know exactly what the world is like when people who are long on absolute, unwavering, unshakeable certainty and short on cautious critical scepticism get their way.
* "I do know with absolute certainty that we would not exist without God," "I am certain that [extraterrestrial life] will only exist if God wants it to exist," "I have no need to explore other religions and belief systems because I have found God" and so forth.