Though I'm often thought of as a grumpy guy, I generally try to do the right thing in my day-to-day life. I don't murder people, I try to be calm and pleasant to my fellow humans. I don't steal or commit adultery or any of that kind of thing. I do this because it seems to me to be the most pragmatic way to live my life. I could maybe take it a bit further and give up beer or sex on the hope that it would have some generally positive affect on the planet, but that's not a risk I'm prepared to take.
Some folks I know also live their lives in a similar way. I guess most nice people generally do. However some of these nice people live their lives in a well-intentioned way because it is prescribed for them in religion - they might be nice guys too, but religion plays a key part in defining their behaviour.
Perhaps they're following some religion from the middle-east and coveting their neighbour's asses is prohibited (for the record I've never checked out my neighbours ass, but it's statistically likely to be OK), or perhaps they're particularly hostile to idolatry or homosexuality. Perhaps they follow an eastern religion and they're doing a bunch of mediation to reach enlightenment. Their behaviour (minus any oddities about hating people based on their sexual or religious preferences) generally looks and feels much like my behaviour - they're generally trying to be nice to others so that we can all rub along nicely on this small temperate rock we call home.
The difference is that the religious don't deviate from prescribed doctrines because that would put them at odds with their faith. Where deviation occurs it may be described as sacrilege or otherwise be used to distance the deviant from the mainstream faith and pillory them. I find this unappealing since in general people are trying to live life in the best way they can, and the philosophies underpinning religious practices should support that.
In my professional life I see similar situations. I work for a company (ThoughtWorks) who are aggressively agile, to the extent that our chief geek was one of the authors of the agile manifesto and working agile is the preferred engagement model for our consultants. In recent weeks both Joel and Steve have put the boot into agile, and though it made me a pariah internally I openly admitted to finding the postings insightful and funny.
It seems I do not have the agile religion. This was probably apparent when I joined ThoughtWorks two years ago. I asked some of my interviewers what this "agile" thing was about, and was told that it involved using techniques like sensible planning, source control, and testing to take the risk out of software delivery. Coming from a middleware background where we had (somewhat) sensible planning, (plenty of) source control, and (the most sophisticated) testing, I found this explanation confusing because the values portrayed under the agile banner were those same values I had held true in an un-agile workplace which the agile religion of course doesn't allow.
I still like source control, testing, and sensible planning and have refined those techniques considerably under the mentoring of my ThoughtWorks colleagues, but I am not agile. I am not agile because I don't believe in the agile religion and I don't accept its dogma. I like the engineering and planning practices that agile teams use - in the same way that I like people who do nice things (even when they do it because of fear of divine retribution). The difference is I don't want to be constrained by dogma into only doing those sensible things which are prescribed by agile. In the same way I don't like being prevented from doing sensible social things because of religious beliefs.
For those that have the agile religion, it is routine to express to others why they are not "agile" because they're not doing precisely 72.799 hour iterations, or because they have code coverage of less than 83.002% or other arbitrary measures. That is counter productive. Instead we should be encouraging good behaviour as a general practice, not enforcing it through religion and the humiliation of non-conformists that most religions - including agile - engender. Sensible software engineers do the things that make projects successful and we should maintain that, but without the dogma.
This is why I am an agile atheist.