How many of them were ever God's to begin with?
Starting from Puritan New England, probably most of them. Have you ever read a Puritan sermon? There is zero equivocation in their writings, they meant every single word, and they were the generations that produced the men who fought and died in the 1776 War for Independence -- they were not joking, not even a little.
Many churches have been part of the problem since the fall of Rome -- during parts of history, most of them.
And before -- the New Testament itself has tons of warnings against the most senior churches (see Revelation 1-3, for example). The church has been broken from day one. But God has determined to work through it
anyway.
Even the ones that aren't get suckered into being part of the problem part of the time. I still remember a Zion propaganda Seder meal program in the fellowship hall of a normally fine Presbyterian church fifty years ago.
Whenever you see a large infiltration of the demonic realm into the visible church, it's a trap. For them. The hook has been planted and God is just letting the reel out.
Every televangelist channel is paid commercial programs from Israel, with something other than love.
You don't even have to dig that deep to see that they are antichrists, just listen to pretty much any clip of them speaking. The devil couldn't make his agents any more obvious if he tried.
Nobody expects the Spanish Inquisition. But a big part of why churches lost their place in society is because a church isn't God, it's fallible men trying to grasp a path to God.
As if God is not wise enough to anticipate and correct for even that. The church, for all its flaws and failings, is God's instrument of spiritual warfare on earth for the duration of the church Age. It is made up not only of "fallible men", but sinners saved by grace. It is, in its very essence, imperfect. But it is perfectible, and the purpose of the sanctifying work of the Holy Spirit is to create from the most improbable group of men, a divine weapon by which the Lord Jesus Christ will strike down the Antichrist himself.
And it has long been a severe temptation to provide parishioners the illusion that they're in the club, rather than providing them with help and guidance walking the rigorous path to the strait door. Because, after all, the customer is always right.
For sure, commercialization of religion has "Anglicized" the church in America. Certainly by the 2nd half of the 20th-century, church had become mostly a way to social virtue-signal more than anything else. It is the inevitable metastasis of success and the devil has very carefully cultivated that cancer. Jesus already knew this would happen 2,000 years ago when he was opening the minds of his apostles to the Scriptures after rising from the dead. It's something we fight against, obviously, but it is no existential crisis for the Gospel. We were never trying to "make the world safe for the Gospel", that's impossible.