Atheism

During the 20th century, atheism wasn't really a thing except for a few scientists and STEM thinkers. (There was the irreligious, but openly admitting you didn't go to church or care much was frowned upon). All the different Christian institutions diametrically opposed each other, even more than they opposed Islam or Hinduism. Jews were tolerated but disregarded as doing their own thing. And they were going to Hell anyway.

(Oh and there was the Church of Religious Science, which was essentially church for atheists and people who didn't take church seriously. There were also liberal Christian faiths like the Unitarians and the Universalists, who eventually combined into one organization. Most people who might have been liberal Christians these days are instead atheist, agnostic or irreligious. Liberal Christianity still exists but it has a tiny following.)

All the mainline and fundamentalist churches believed as the Roman Catholic Church did, extra Ecclesiam nulla salus. No salvation outside [this] church. Seriously, the Presbyterians thought the Lutherans were going to Hell. The Lutherans thought the Pentacostals were going to Hell. The Pentacostals thought the Baptists were going to Hell and vice versa all around.

Then came 9/11 and, inspired by that event, Richard Dawkins and company founded the New Atheism movement, and so in the aughts, the religious ministries had two new enemies, Islam (mostly radical militant Islam, but the churches didn't distinguish) and rapidly-multiplying atheists, eager to meet their religious brethren on the debate stage.

And suddenly, all the churches were much more tolerant of each other, at least when facing the public. Then there was only one faith: Christianity.

Within their own chambers they still felt the other denominations were deliberate deceptions of Satan, and destined to hellfire, but faith leaders speaking to the public would presume that when they preached (whether condemning Muslims or gays or women or secular society) they spoke for all of Christendom.

And now that's a problem for every denomination that isn't white Evangelical Christian nationalism (which is bleeding out of the US into the rest of the world), since all the more beatitude-minded faiths -- including some evangelists -- don't want to be represented to the rest of the world by Paula White-Cain and Franklin Graham and Robert Jeffress, or for that matter, Jesus Donald Trump.

In fact, Christian scholars are terrified that the current Christian nationalist movement (which actually extends into other denominations, including the Catholicism of all six of the conservative SCOTUS jurists) may end Christianity's reign as a major world religion.

Still, if the ministries were successfully able to chase atheists back into the closet, and non-Christians out of the US borders, they'd just turn on each other much the way Protestants and Catholics warred on each other bloodily across Europe.