Growing up in an ostensibly reform-Jewish household that didn't even take that seriously, atheism was easy for me, so I don't know how hard deconversion is, how much it hurts, or how much of one's entire conception of self is trashed in the process and can't be recovered.
As an atheist, it's tempting to say, "Look, it's not that bad: God doesn't exist exist, but you can still go to church and praise God and stuff if you want; it's just that there are benefits to being honest about what you're actually doing and why."
Somehow, I suspect that this is not a very convincing sell.
Applications to other topics are—as always—left as an exercise to the reader.
As someone who grew up catholic and really believed (indeed, as a catholic that was probably my problem...I think only that small fraction of catholics who go to church as 6am every day really believe) I can assure you that you're correct and that this is about the most ineffective deconversion pitch you can imagine. I realize you already know this (so what follows is not an argument) but maybe if I offer my perspective it might help you to be less tempted to say that.
First, even though causally it may be the social and ritual aspects of worship from which people derive benefit theists don't think of it that way. EVERYONE groans about getting up and going to church and most people pray as something of a duty (they may find it comforting too but just as you might feel better after daily meditation doesn't mean you don't have to discipline yourself to do it). So telling someone 'hey you can still go to church and praise god and stuff' is like arguing that someone's diet won't work and then reassuring them 'hey you can still eat brussel sprouts.' Yes, some theists (though not US catholics) appreciate the social benefits of going to church but to the extent that is important to them it's as a way to associate with likeminded people (otherwise they could just go to a bar or book group) and that is no longer available to the nonbeliever.
Secondly, as a reform Jew you are unlikely to appreciate the extent to which non-believers really can't continue to go to church and openly worship. Reform synagogues (and if not them then the social expectations of the attendees) emphasize the forms of worship and encourage participation by those of Jewish heritage who don't believe or are uncertain. Most christian religions emphasize belief. The cultural context makes it clear that by engaging in these open displays of faith one is professing belief...and to do so once convinced god doesn't exist (not just doubting but genuinely no longer a believer) would feel like a lie.
I feel like the appropriate analogy would be to imagine telling a gentile, 'Hey, don't worry about being a gentile or about converting. Just study up on the Torah and Jewish rituals and show up at your local synagogue. If you pretend no one will ever know.' Yes, in a certain sense it's true. If you were willing to deliberately mislead the other parishioners and lie when questioned you could fit in fine. However, that kind of deliberately deceptive behavior would undermine many of the benefits that going to synagogue offers. A similar thing is true of continuing to attend most christian churches while truly not believing. At the very least one has to avoid questions that would force you to either lie or admit your non-belief least one be socially ostracized. No one would object to your presence but you would feel like an outsider (much as one does as a non-jew visiting a synagogue).
Finally, and most importantly, I think many theists (like I did) find belief itself to be the most important benefit. Yes, abandoning what everyone around you thinks is correct is hard (though my grandfather was effectively an atheist at the time which made it easier) but giving up the idea that the universe cares about you in an individualized human way and that you will be rewarded for your good deeds was just as hard. Especially when things are tough it is comforting to believe that your suffering is being appreciated and you will eventually get your just deserts.
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Far better to tell the
Oops, I left off the most important part:
Far better to tell the theist this:
Look, the very fact that you are having this conversation with me and taking it seriously means you probably don't really fit in at your church (some bay area churches excepted but some of them barely qualify as Christian). You probably already feel a little left out of the fellowship lots of your coreligionists claim...you have (or at least are developing) doubts that you can't voice with them and questions they don't want to talk about. There are other people like you and it's far far better and more enjoyable to join a group of likeminded people who will enjoy talking and debating with you over these issues. If you still have religious impulses so much the better, rationalists get bored discussing these issues only with people who agree with them. Also rationalists don't insist you get up early on sunday to hang out with them.
Point out to them that all that guilt they feel over not really believing or not following the teachings of their church will disappear once they make the leap.
At least for me the harm was just in the agonizing wait to leave...once one makes the leap it's a lot better and you should tell them that.