TL;DR: Deconversion, expatriation, integration, direction.
There are atheists who are also anti-theists, taking the position that a religion's adherents should decay into atheists, with the religion itself fading away. Some might argue for cultural remains.
One could also reflexively agree with the notion that religion is the opiate of the masses (Karl Marx) and say that some form of neutered religion is necessary for "those people", taking a position that humans need some flavour of "bullshit". This is in between the obsolescence and discarding of a religion in whole and a religion fading into cultural memory.
I see the rise of new age woo woo or spirituality as being something like this. It's the decay of the greater coordination of religion into the personal desperation for a connection to "the divine" (or whatever).
REDACTED an entirely imaginary religion which is fundamentally hostile to human rights and the comforts of contemporary civilization, where it would be in the greater moral good to oppose it, one could wonder about taking an anti-theist position.
Should there be a position to deconvert such a.. purely theoretical religion? Can that even be done? What would that look like?
Moderates cannot overtake a religion with a fundamentalist majority in power. I mean to say that within the ranks, within the imaginary "borders" of a religion, it cannot change itself without an undercurrent of change then a spark which ignites an open rebellion. It definitely isn't possible for the rebellion to begin top-down. The peasants with torches and pitchforks would kill them, prompted by some other body who would themselves take the vacated positions.
This means that atheists, even with the expertise of being apostates from that religion, are not useful at all in any form of moderating, neutering or dismantling of their former-religion. Not from the inside.
So what could be done, by outsiders, to stem the ongoing threat of such a religion?
Promoting a "civil war" within its ranks, where denominations oppose one another, might be considered useful, especially to distract the extraordinarily-dangerous ones. Dangerous, as in active and effective terrorism or even the vaguest access to weapons of mass destruction.
A civil war brings grave risk. Such an internal struggle would be thought of, in their minds, as having the goal to determine the "correct" denomination. The nightmare is with one of those parties winning. One might think that even in this case, should it even happen, they would be weaker than before a civil war, but they would be immeasurably stronger in their moral perspective.
Atheists in such a civil war would find themselves targeted by all denominations, each hardened by the righteousness of participation, to the point that there may be a cease fire between them to take on their common apostate foe. Outside encouragement by deconversion could backfire with such force as to quash the greater denominational conflict.
A civil war begun by either its internal participants, or instigated and encouraged from the outside, would create a situation which would have to be taken advantage of by outsiders while it is alight. Simply waiting for a victor does not solve the problem.
For outsiders to protect themselves from such a threat, sparking such a conflict might be thought of as a good idea, but it brings complications.
Vague-participants, what one might mistakenly call "civilian", would flee struggle. They are less or not at all committed to participating, including apostates who "don't have a dog in that fight". A simple deconversion cannot predict an atheist's participation in some sort of undercurrent of struggle within the region of their former-religion, especially during a violent open civil war.
Given a civil war that is not military in any sense, staying social, political and peaceful, apostates would still be in a very weak position. Even in such fairly-peaceful circumstances, when given any chance to exit a timid conflict, many will leave if they perceive enough threat to them or a need for effort by them.
For apostates to stay and fight would be a balance between risk and reward. Even the most peaceful struggle may not be worth fighting if there is enough of their culture for them to trivially resettle to. Simply appealing to their expertise or morality is meaningless compared to the fear that their greater culture exists only within the throes of that civil war. If they value their culture enough, and it does not and cannot be made to exist elsewhere, they will stand and fight.
Those who cannot leave may still not be forced to participate. They will be either be brought into the war under duress, faking a denominational leaning, or will withhold their participation and watch their perspective stay individual and never find a community of their like-minded peers.
Beyond a mere complication, it is a serious problem that non-apostate denominational civil war participants will reach out for resources, including and especially in the form of new converts from "the outside", spreading the madness of their civil war. Those exiting are not just the moderates and apostates resettling to avoid conflict, but fundamentalist and extremist missionaries. The notion of instigating a civil war to protect outsiders of a religion spreads the conflict like a fire.
Furthermore, should outsiders ever be blamed for starting or participating in a civil war, including picking sides such as instigating or supporting apostasy, then any and all outsiders become targets for the self-perceived righteous. The instigating outsider may pull their non-participating peers into a greater conflict. Just as fighting apostates becomes a common enemy for all denominations, outsiders can similarly become a common enemy, undermining the efficacy of denominational infighting.
Containing a civil war would have to be unilateral. There can be no way to differentiate between apostates, moderates, fundamentalists or extremists. They may not even be able to tell one another apart until there are enough that they would "set up shop" in communities, support groups or begin proselytizing for additional members. Or, for extremists, arming themselves. The problem becomes much greater when such people are thought to be integrated. Perhaps they are legally integrated as citizens, and become afforded common protection. In such a case, any hope for their "containment" has utterly failed. At best, they could be placed on a watchlist.
With containment failure, the concept of borders to a religion are confirmed to be undefined. Understanding this from the very beginning is usually impossible, with the earlier position of outsiders being that of not recognizing or working against a problem until it actually becomes a problem that needs attention.
When the concept of outside and inside or strict religious regions no longer exist, as is almost always the case, and where containment is naive, a "buffer" of apostates might be considered. That is, deconverting those who have either naturally migrated from or have fled from a religious region which is either stable or alight in civil war. However, consider this not just as working to deconvert moderates, but also include attempts to downgrade the threat level of extremists or fundamentalists by somehow turning them moderate or apostate.
There are historical examples of a religion naturally cooling into moderation, and outsiders might mistake any religion as inevitably doing the same. They may think that the borders of a religion spread into theirs and their migrants cool and integrate as moderates. This is an extraordinarily dangerous assumption.
If the inevitability of cooling is recognized as unreliable, then imagine somehow diminishing any current or future threat by intentionally "fuzzing" the borders of a problematic religious region. This doesn't need to be a geographic concept, but can be thought of as fuzzing the hard differences between fundamentalist or extremist "insiders" and the moderate or apostate "outsiders". This isn't to say that the religion needs to be integrated or some concept of tolerance needs to be pursued, but to describe a sort of thickness of boundary between the inner most-dangerous part of the worrisome religion and its outer least-dangerous part which fades into the culture being protected.
One could continue to examine these ideas with the analogy of a fire. Sparking the fire to begin or promote a civil war has problems. A fire might eventually burn out, but fire spreads. Attempts to contain a fire might make things worse. Having "moderate fire" around its edges doesn't make a whole lot of sense.
Thinking of apostates as being anti-fire, perhaps as water, might be a useful addition to this analogy. Throwing a little cup of water into the middle of a fire will have it attacked by all sides, but having it around its borders, especially where it is weakest with so-called "moderate fire", might be a good start.
Apostates could push against moderates, helping water-down moderates into more apostates. They could water-down fundamentalists into moderates, and extremists into at least less-threatening fundamentalists. They could also help outsiders.
Think of apostates not just as soldiers fighting against their former religion, but also as helpers of those outsiders to be protected. Apostates aren't just the former-religious but truly represent integration into a culture they now care about.
The "not all of that religion are like that", is the position moderates take. This prevents proper recognition of problems and action against them.
Apostates have an immeasurably useful stance. They can say "yes, all of that religion are like that" and have their expertise be listened-to. They can force a reveal of so-identified moderates as instead being fundamentalists or even extremists. This recognition helps bring moral correctness in efforts to protect against a religion. It helps weed extremists from the field of moderates. Apostates can hold moderates to account, and help prevent them from being used as shields by fundamentalists.
Apostates can point out that something is cultural and not religious, and so that thing requires no religious protection. Yes, some cultures, or at least points of view within them, are superior to others. Recall the mention of human rights and the comforts of contemporary civilization. Dismissing the validity of cultural exports from a dangerous religion's-culture allows less risk to be imported into the ones being protected.
Only by dampening the borders around a fire can its spread be slowed and hopefully halted. Only by wetting anywhere possible can there be some hope for the fire to burn itself down. Only by gathering more water can there be any hope for it to impact a fire. Perhaps only by the hand of the formerly-outside culture which apostates have integrated into can they be directed in the most effective way. Perhaps only with help can they be effective.
Light the fire of a denominational civil war then stand well back to not be noticed. Deconvert moderates into apostates. Encourage their use as dogs against moderates, to sniff out extremists. Recognize extremists as soldiers and act upon them as such. Demonstrate to one's people the correct differences between moderate and extremists and the value in joining them. Build a protective barrier of moderates, apostates and one's own concerned forces capable of drowning a weakened set of denominations to re-take remaining cultural and religious heritage and keep it moderated.
The end goal is a religion whose borders are no longer fuzzed out dangerously, but fuzzed-in, hopefully all the way through.
Instead of hoping for a religion's eventual natural cooling, these ideas force it.
So is deconversion a solution for a problem-religion? It is vastly more complicated than that.
Worried outsiders to a problem-religion can use deconversion in their toolkit, but they must also aid apostates through their integration and empowerment.
Finally, there may be no other way to quash the fire of religion than with its atheist apostates.