https://en.wikipedia.org/wiki/Master%E2%80%93slave_moralitySlave morality
Unlike master morality which is sentiment, slave morality is literally
re-sentiment—revaluing that which the master values. This strays from the valuation of actions based on consequences to the valuation of actions based on
"intention".[4] As master morality originates in the strong, slave morality originates in the weak. Because slave morality is a reaction to oppression,
it vilifies its oppressors. Slave morality is the inverse of master morality. As such, it is characterized by pessimism and cynicism. Slave morality is created in opposition to what master morality values as 'good'. Slave morality does not aim at exerting one's will by strength but by careful subversion. It does not seek to transcend the masters, but to make them slaves as well. The essence of slave morality is utility:[5] the good is what is most useful for the whole community, not the strong. Nietzsche saw this as a contradiction. Since the powerful are few in number compared to the masses of the weak,
the weak gain power by corrupting the strong into believing that the causes of slavery (viz., the will to power) are 'evil', as are the qualities they originally could not choose because of their weakness. By saying humility is voluntary, slave morality avoids admitting that their humility was in the beginning forced upon them by a master.
Biblical principles of turning the other cheek, humility, charity, and pity are the result of universalizing the plight of the slave onto all humankind, and thus enslaving the masters as well. "The
democratic movement is the heir to
Christianity."[6]—the political manifestation of slave morality because of its obsession with freedom and equality.
...the Jews achieved that miracle of inversion of values thanks to which life on earth has for a couple millennia acquired a new and dangerous fascination--their prophets fused
'rich', 'godless', 'evil', 'violent', 'sensual' into one and were the first to coin the word 'world' as a term of infamy. It is this inversion of values (with which is involved the employment of the word for 'poor' as a synonym for 'holy' and 'friend') that the significance of the Jewish people resides: with them there begins the slave revolt in morals.[7]
Emphasis mine
Basically, those with slave morality are realizing that democracy is a far more potent tool for enslavement (edicts, laws, etc., control in other words) of those with master morality (businessmen, the rich, right-wingers, libertarians, those who seek freedom from enslavement) than religion. It's just a migration, they're not actually disappearing, only calling themselves a different name and pushing their agenda with a different spin; given Nietzsche is correct, they're adjusting their "re-sentiment" in accordance with modern master values, which is why such incredible, unusual focus has been placed on the gaming community for example, a popular venue for those with master morality;
this comic was made to highlight the bizarre refocusing of values, giving a bit of insight to how "re-sentiment" works (literally resentment).
Nowadays they're called SJWs (feminism being among the most popular, complete with its own pinata of the masters being "the patriarchy"), Marxists (USSR or China being the closest examples to slave morality encompassing master morality), democrats, liberals, socialists, egalitarians, and of course "The Jews" as slander, essentially anyone who identifies with the left. Don't be fooled by the rise of atheism, it's still the same stuff from before Christ just with different wording, the slave-moralists are just catching on that the old methods aren't as convincing and that these new methods are, at least for the time being.
Christianity worked out for a long while for slave-moralists--at some points in time completely overshadowing entire nations--but nowadays it just doesn't work out, master-moralists don't listen to the church much anymore, the church is powerless to help slave-moralist desires. Islam is still a very prevalent method of control in the middle east, however I am not sure how long that will last but it seems to be very much functional like Christianity was in the past (albeit abhorred by competing slave-moralists in the west.) You can tell the Catholic pope is trying really hard to appeal to slave morality but I'm not sure how effective it will be; he's certainly convinced a lot anyway, I have atheist friends who really like him regardless of the religion. Ironically, the pope is the very type of person you'd figure the slave-moralists would abhor, but apparently the best way to succeed in this field is to pretend to give a shit about the whims of the masses (as any politician nowadays could attest to.) The numbers will continue to change as the church becomes less popular with master-moralists.
So the question becomes: why's it getting less popular with master-moralists? I believe this is due to the great swathes of information available, particularly due to the Internet. Master-moralists have a desire to accumulate factual knowledge which helps them visualize and fundamentally understand the world around them, to "see" what's going on in the world, allowing them to control it (different from slave-moralist control which involves manipulation of people, rather than the master-moralist's manipulation of objects e.g. business operations.) For most of history, this was hard to pull off: there was no video, no photographs, no wireless signals or audio, all you had to rely on was books and hearsay, so developing an accurate and complete view of the world was very difficult and usually skewed to the point of stupidity (re: earth being center of solar system.) Nowadays it's easy as pie, there's so much content to fill your brain with, and the best part is we all share the same Internet so we all have access to the same stuff, allowing the best (i.e. most relevant and accurate) knowledge to rise to the top, and the rest to fall to the bottom. Essentially, the entirety of the developed world's master-moralist population is steadily coming to the same conclusions on things, and these conclusions are shared, refined, and becoming deadly accurate to the reality of things.
The more perspectives you have (i.e. having listened to the perspectives of others, esp. those with very developed perspective), the clearer you can "see", your big-picture vision; a part of this global common understanding process is the rejection of religious principles, as the Internet effectively makes it obsolete. Used to be, you'd get your understanding of the world through religion, which would always be far more developed of an understanding than you'd come up individually or within your limited community, or however far a letter could reach, however many books you could get (harder to get the further back in history you go, allowing the likes of kings and emperors actual advantage in the world as they'd always have an infinitely greater perspective than anyone else.) In other words, people's perspectives were harshly limited, making it impossible to formulate advanced thoughts (well, what we'd call our thoughts.) As communication developed, so was our ability to formulate an idea of the world, giving us a mental "hill" the brain sits on which grows taller every time it learned something new and saw something as someone else saw it, giving a greater and greater vantage point over the world.
Then came the Internet, and people started to communicate, with words and images and videos and audio, more and more people participating all the time, sharing their knowledge and sharing their perspectives, broadening their views far greater than most anyone has ever had in the past, allowing the greatest amount of people with the greatest amount of perspectives to multiply their "visions" and come to similar conclusions. It's a slow and messy operation, and there's a lot of work to do (and undo), but with this constant re-envisioning of the world, of expanding one's mind, inevitably the individual is compelled reevaluate their religious beliefs, which is bound to cause more apostates than having few feeling need to reevaluate resulting in stability in general belief (what one may call stagnation in belief development.)
We're becoming more and more clear and critical in our thought as our ability to form realistic understandings of the world improves, and it's causing very rapid and very structured changes in its participant's beliefs, which in turn effects even non-participants ("internet leaking into the real world".) Religion is becoming unpopular because it's no longer an effective method of control; it's been unpopular for some time, the modern state has long been a replacement for it, there's just lag in between realizing you've abandoned your religion and actually abandoning your religion. I think it's reasonable to expect the popularity of religion to continue to fall; the cat's out of the bag by now, the religious perspective once reigned king in common perspectives when acquiring a complex perspective was difficult, but now it's so easy to have your beliefs expanded and challenged by someone operating outside the boundaries of your perspective, it's that much harder to just stick with a single belief for long.
Gotta keep up with these fast and strange times, it's of extreme importance for a belief's survival to change when it's been challenged on something believed to be untrue. Religion just ain't gonna do it, nor is it standing up to the tests of a super-perspective, really shining the light brightly on its flaws and strengths, as it shines the light on everything; only those beliefs which aren't afraid of the light will survive, i.e. only the most consistent and accurate and true. And of course, their respective shadows, the slave-moralist's re-sentiment, which will prevail simply based on which is the more subversive hate group.
