It's an interesting question. Indeed, why do Christians de-convert? I do not suspect the answers to be easy to come by. Even so, this subject has been raised on a Christian site I frequent, and I thought it worth discussing here.
According to one Christian:
Interestingly the fact that many would walk away from their faith is anticipated by Jesus Himself. He gave three reasons why people walk away from their faith:
1. Spiritual opposition.
3. The desire for material pleasures. – Mark 4:13-20
These reasons comport with observations I have made with many interactions I have had with atheist converts. They never merely reject Christianity but they always reject Christian morality as well. It’s never a case where they persist in the lifestyle Christian beliefs require (sexual purity, self-control, self-sacrificial relationships, etc) and only reject its truth claims. And it isn’t necessary for this to be so; they themselves often argue that they are equally moral to Christians. And yet they invariably adopt lifestyles that are morally antagonistic to Christianity. I think it is no coincidence that atheist converts are mostly young men whose lives are most driven by their selfish passions, and who are most willing to subvert belief to desire... The ‘need and desire’ Christianity doesn’t fulfill, and can’t, is the freedom to sleep around guilt free or live a lifestyle that is gratuitously selfish. And as much as these desires drive the choices of young men they provide a strong motivation for rejecting Christianity. I also note that a lot of these guys when they get older and marry and have children are much less antagonistic. They may not return to Christianity, but they certainly don’t see it as the enemy they did of their youth. This isn’t universally true but is often the case for men I see in committed long term relationships with healthy families.
In the end I can only speak from what I observe – but as the intellectual case for materialistic atheism seems to have uncontestable [sic] weaknesses and Christianity is more than rational in it’s understanding both of the natural world and as a foundation for human flourishing, I am inclined to conclude that rejection of Christianity is more often a product fulfilling one’s passions than it is of intellectual satisfaction.
This however, I find, is limited in scope. Personally, I think the criticism may be applicable to some atheists some of the time. But after reading through the list of (Biblically supported) reasons for why some Christians *think others deconvert and leave Christianity, the more I find that either I am a grand exception or the above generalization isn’t encompassing enough.
Contrary to the above opinion about atheists and their wild ravenous, lusting, sexual promiscuity I have been with the same woman (my wife) for eight years (and we've been happily married for four of them). We have a beautiful daughter. And I don’t have the time nor luxury to sleep around because of my devotion to my family. Not that would ever want to.
My deconversion hinged on my love and passion for Jesus and then, equally, my love and compassion for my wife and who she was as both a Japanese and a free thinking secular Buddhist.
First, in my Christian youth my passion burned for Jesus, I was filled to the brim with the Holy Spirit, I was a Campus Crusader for Christ, I was a Bible camp counselor, was a youth leader in my local church, was part of numerous Christian charities, I helped organize and partook in various youth retreats with the aim of enhancing the bonds of Christian fellowship across the U.S., I wrote Christian apoplogetics on my blog called “The Chronicles of a Sympathetic Christian,” and so on. All this was because I burned with a passion to bring the love of Christ to others.
My zealousy got the better of me though… because I wasn’t satisfied with the stained glass, pristine, Jesus which was being preached from the pulpit any longer. Even then I knew that such a figure was dressed up and/or molded to fit the pastor’s sermon. I knew deep down that such a personage was largely artificial–a Jesus partly evolved from scripture and partly from the collective imagination (i.e., parochiality) of our own devising.
I felt the Holy Spirit compelling me toward a more intimate relationship with Christ. Therefore I embarked upon a personal spiritual journey to enhance my understanding and grow in my relationship with Jesus. I began by pursuing my desire to have the most intimate relationship possible with my Lord and Savior by learning about the real historical Jesus, the authentic man behind the Gospels, not the watered down Sunday school version. I was bound and determined to learn everything there was to learn about the Gospel Jesus.
Approximately 120 history books later the Jesus of history proved to be much more illusive and problematic than expected. In fact, the Jesus I knew and loved was not the same man as the real historical person. Not even close. The Jesus behind the Gospels (the Jesus which supplied my faith meaning) proved a romantic ideal, meanwhile the historical Jesus became impossible to demarcate. Suddenly the Jesus behind the Gospels vanished and like a sand castle on the beach washing out to sea.
Don’t mistake me, however, I’m not saying there wasn’t a real historical figure called Jesus the Christ. In fact, I believe there is enough internal evidence in the Gospels to make the case that there was a real person–but again, as I stated earlier, it’s relatively impossible to demarcate the historical Jesus. This means we can’t really define the Gospel Jesus as historical, since we don’t know what historical bits to delineate from the non-historical. That’s one of the biggest concerns I have as a historian.
After that it is just a matter of familiarizing oneself with early Christian history. After which, I think all the clues point to a literary hypothesis based on a legendary figure. This mythologization is traceable, unlike the historicity of the real personage.
At any rate, just as soon as I realized there was no tangible figure to base my faith on, along with the cognitive dissonance certain analytical and historical concerns raised, it dwindled to practically nothing. Even so, my passion of getting to the truth outlived my faith. As it turns out, my faith failed me, not I it.
The second part of my deconversion was more involved as it involved me discovering that there was no genuine moral basis in my Christian belief system. The catalyst for this realization was me meeting my wife and her being of a different background, both culturally and with regard to religion as well. This of course raised other forms of cognitive dissonance, mostly dealing with the moral dynamics of a secular worldview vs. my Christian worldview. This raised new philosophical concerns I had never had to consider before, and it forced me into a very serious Outsider Test of Faith, of sorts.
To make a long story short my Christian moral precepts did not stand up to exacting scrutiny either and yielded no answers to the sorts of questions I was asking. Yet since my OTF involved real world consequences, I had to find a better more evolved, philosophically sturdy, moral system (or systems).
Interestingly enough, many of the Christian values I held were already inherently a part of other belief systems. It was only a matter of assembling the best moral theories (admittedly a work still in progress) and getting mainly the same moral results minus the limitations of Christianity.
So you see, my journey from belief to nonbelief was mainly intellectual, but at the same time, there were emotional factors involved. Even so, most of the emotional factors were falsely presumed, i.e. I realized my wife's love was more real than the love I thought I was receiving from Christ, and then keeping this love depended on me learning to love--and frankly, my prior Christianity stifled any attempts to broaden my worldview and love others unhindered by theological concerns and dogma (granted more liberal Christians typically do not have this worry--which is why I suspect Fundamental evangelical Christians, such as I was, have a higher deconversion rate than liberal Christians and even Catholics). In a way, of a sort, I did end up rejecting Christian morality--but not because I wanted to live a heathenish lifestyle or anything of the sort. Rather, it wasn't compatible with my desire to encompass a broader worldview which allowed for a great compassion and understanding on my part--Christianity was just too insular and its morality was too primitive (i.e., reeked of tribalism of a goat herding age--which is to say Christian morality is entrenched with a simplistic Bronze aged mentality restricted by conventional dogmas).
Well, that's my deconversion process in a nutshell. Granted, the finer points of the arguments which ultimately convinced me atheism makes more sense than theism are much more nuanced, naturally. But even so, as Paul Harvey would quip, now you know the rest of the story.