I don't know about the US, but in Canada, it was a loss of doctrine, and stepping away from believing the Bible.
It started somewhere around WWI here. The Methodists and Presbyterians decided to "unite" and form the United Church of Canada. Presbyterians are Reformed, and Methodists are Arminian. So something had to give and it was doctrine. By the time I attended Sunday School in the United Church in the mid 1960's, in grades 4 and 5, the Bible was not used at all. I have no idea what we were taught. Always interesting that the Bible verses I learned in Baptist Sunday School from grades 1 to 3, I still remember.
So the first apostate church. My husband's aunt was in the United Church in the 1980s and 90s. The national publication was a mass of conflict between conservatives, who believed Jesus died for their sins, and liberals, who thought Jesus was just a man who came to show us social justice. I went out with some high school friends about 5 years ago, who were still attending the United Church. I said what a mess the church had seemed to me. They laughed and said it was all good now, since they got rid of the "trouble makers." Can you say social club? Our interim pastor last year stayed in the United Church to try and reform it from within. When they voted not just for gay marriage, but for gay ministers, he spoke out and got his ordination yanked.
This same thing happened to the Unitarians and most Quakers. Just follow the Spirit without the balance of the Word of God, and BAM! You have a totally apostate church.
I do agree we need the Holy Spirit. It isn't a church without the leading of the Spirit. But neither is is a church, without the balance of the Word of God, and people living for Jesus. There are a lot of supposed Holy Ghost led churches, which are just as much social clubs as the United Church in Canada. I remember one large Pentecostal Church we attended. Lots of outreach, but no results No one prayed, and although a lot of lip service was given to them leading of the Holy Spirit, there was literally none. It was a nice big, comfy-pewed church but no Spirit.
You cannot have a vital church without the Holy Spirit, nor the Word of God, properly exegeted. Good preaching has been the hallmark of the Baptist churches I have attended. When one pastor went astray, I told him I was tired of his bad preaching, and left. He was unrepentant. He had a lot of plans and ideas for reaching the lost. I applaud him for that. But no one got saved! So, really a lack of the Holy Spirit, for sure. Our new pastor we just called is also really off the beaten track. We had a traditional old retired Baptist preacher yesterday and the Holy Spirit fell. I was dancing in the aisles. Well, as well as I can dance with my feet and knees. Not because I was showing off, because God gave me the freedom and the pastor had really preached on how to release the Holy Spirit to move. But always, within the context of Scripture.
Because if anything goes against the Bible - be it doctrine, or worship or methods of evangelism, it is not from God and doomed to failure.
Anyway, that is my experience.