What happened to Christianity is BIOLOGY.

Our biology evolved over billions of years to fit a pre-civilization environment. When we invented civilization we radically changed our environment, creating a mismatch between our biology and our environment.

Evolutionary mismatch is the cause of all extinctions.

Unlike all other human species, sapiens had developed a workaround for the threat of extinction, which is our capacity for complex culture. We could modify our behavior relatively rapidly through cultural means instead of having to wait hundreds of thousands of years for our biology to evolve.

Every human species that did not develop the capacity for this counterbalance to evolutionary mismatch is now extinct.

Whether consciously or intuitively, early humans sensed that their fluid capacity for culture was what was saving them from eternal destruction, and so ritualized aspects of it and regarded it with the greatest of reverence.

But just as biology is much slower to evolve than culture, it is also much sturdier. When biology and culture compete for control of behavior, biology has a persistent advantage.

As long as cultural counterbalances were free to evolve as conditions changed, they could compete with the sturdier opponent. But when writing was invented, the culture, already regarded as sacred, became ( relatively ) frozen in time. It was seen as too sacred to tinker with.

At this point, culture’s meager capacity to compete began its fatal obsolescence. If change was possible at all, it was difficult and costly. Religion has lurched along under the constraints of the written word and the assaults our rebellious biology delivers ( misappropriation, misunderstanding, corruption, etc. ) but appears to be losing the battle wherever the adolescent science of reason is dominant.

This is not the fault of reason, but of our immature capacity wield it.

Now we are in a race between the decaying of our old system of extinction avoidance and the maturing of our new system.

If we are not to follow our Neanderthal and Denisovan cousins, and the six others ( that we know of ) who vanished as civilization approached, we will need to be careful what workarounds we throw away before we learn how to replace them. Our own biological nature is now the greatest threat to our continued existence.

Nothing happened to Christianity, it was ALWAYS a militaristic, master race, political, violent death cult.
During the mid 20th century, the churches, the major ones at least, distanced themselves from public violence, because two world wars and the rise of fascism showed them up as being to akin (and publicly supportive of) totalitarian doctrines.
So for a period of about thirty years between the 40s and the 70's the churches became the church of "Jesus Christ meek and Mild" instead of "Jesus Christ, warrior messiah of the last days"
But as right wing politics Thatcher and Reagan for example began to become popular again in the 1980s, the churches seized the change and fundamentalist religious right rose in popularity again and the
charitable loving Jesus once more gave way to the "Take up thy bed and walk AND GET A FUCKING JOB" Jesus of the likes of Norman Tebbit and other right wing extremists.
The children of the 1980s grew up to be the Boris and Trump voting neo Christian bastards of today.

The world changed around it, especially in the democratic countries of the world, where governments realized that they had to provide care, and express a social conscience to really earn votes. While secular charities grew more important, as internationalism meant that the charities had to operate across state and cultural divides, and the world as a whole embraced ideas like human rights and environmentalism, even if it did not always agree on details.

The religions (all of them ) therefore lost their monopoly on good values and social conscience, which they once had when the state especially meant, simply the biggest and most evil gang of robber barons around, as it did in the middle ages and the age of empires. For religion has only ever thrived by providing an alternate voice to the state and the mainstream. Without that, who needs it ?

Religions therefore have had to find a new audience, if they wanted to continue, and they found that audience in those criminal and anti social elements who were finding themselves no longer able to express things like class, racism and sexism in mainstream culture. And as the criminals move in, the good Christians, Jews, Hindus etc., especially the moderate ones, leave. Which then leaves the criminals even more in control, thereby driving out even more moderates, and it then becomes an unstoppable spiral downwards.

The fact is that religion only really ever existed, mainly as an alternative to the state and the popular culture, and as those turned to the light, well one player in a game has to play the dark side, or quit.

All religions for a short period may imitate the cultural values but always will bounce back to the damn original, you can see the same pattern in all of them. Unfortunately, those periods gave the wrong signal about the reformable capacity of religions.

I had'nt thought of the time from when I was growing up to now as a lull in christian extremism, but it does make sense. About 10 years ago I was dating a Polish lady. Catholic. She had actually received communion from the then Bishop of Krakow, who grew up to be Pope John Paul II. For some reason, trying to be the nice guy, I went to Easter Mass with the family. The local priest gave his sermon to close the Mass. Half way through, he got to talking about "threats" to the church, specifically mentioning Militant Islamists, and Militant Atheists. I would love to have a video of my head shaking back and forth in a "did I really hear what I thought I heard". This priest was so terrified of losing his influence, that he would say anything.