great point. I wonder why it is so easy for many people?
I know for me, I slid into atheism/nihilism probably junior or senior year in high school. Prior to that, and especially in my childhood years, I was a devout Christian. I come from a long line of devout Christians and have several ministers in my family. My father is the most faithful Christian I know. His faith has an almost child-like quality, at least partially because he cheated death a couple of times (in military combat and on account of a major health issue that kills a lot of people).
The tipping point for me came when I read The Stranger by Albert Camus for English class. The second half of the book consists of a lot of philosophical/religious dialog between a prisoner and a chaplain. It was the first time that it dawned on me that my religious faith was a choice I was making. I never realized I had a choice until then, which seems odd to say in retrospect, but is the truth.
Why did I switch to atheism at that time?
Obviously broad socio-cultural issues were at play -- atheism was and is a visible option and, at the time, perhaps I felt that the people I aspired to be like in my life were atheists. (I think I started smoking cannabis later for a similar reason -- I wanted to be an artist and I thought that's what artists did.) I think there was also some typical teenage rebellion at play.
Chapman makes the point that "stances" tend to be unstable. At some point, my religious faith reached a crisis point. Looking back now, I believe the crisis may have started earlier, when I discovered sex, and that really didn't square with the conservative denomination my family adhered to. Also, the denomination of my childhood is a literalist denomination, and that became intellectually difficult to square with what I was learning in public school.
I think it's pretty common to throw the baby out with the bath water in one's first personal religious rebellion, so I went full atheism instead of "shopping around" for a different spiritual approach.
I also think it's common for folks to fall away from religion and bounce around a bit. I have toggled back and forth between atheism and spiritual belief for years at a time in my adult life.
It was hard for me to go back to religion after being an atheist. I think I felt somewhat duped by the religion of my childhood. Possibly I felt humiliated. How could I have been so foolish to be taken in by those beliefs for so long? -- that sort of thing. Possibly I felt resentment at my father for making his belief a priority above other concerns.
According to Chapman, our stances or postures about questions of meaning, purpose, values, ethics, etc often times come into crisis when the stance we are holding dear (fixated on) faces some big challenge, either emotionally, intellectually, ethically, etc--we learn new information that makes it impossible to keep holding the stance, or there is some ethical issue that arises that we need to work around, or, if we're in a nihilistic stance, we feel emotionally bankrupt and we need more nourishment, or some combination of those factors. When those types of things happen, people change their posture.
I know from my own personal experience that I have changed postures over the course of years a few times, but I also change postures in little micro-bursts when I cycle through and consider different models of the universe, consciousness, mystical experience, etc.
Chapman postulates that there's a complete stance that is at the center of all the other stances, and the key to the complete stance is that it accepts a degree of "nebulosity" (mystery) along with any sense of "pattern" (meaning). But it is hard to stay in that stance for long, because it too is unstable, and so even folks who are interested in his model will find themselves bouncing around the different stances from time to time.
BTW, the stances are more about "soft" topics like meaning, purpose, values, ethics, etc. I'm not quite sure what he makes of questions about "scientific" knowledge. That's where the Skeptiko questions about consciousness and the NDE research get interesting -- I'm not sure how he'd answer those types of questions. He maybe would just punt and say that consciousness and life after death seem to be intractable questions, and he prefers to focus more on the postures toward the answers and how those postures work within us rather than focusing on the "provisional" answers themselves.