I think it was very gradual, and I see three phases of this. In my opinion, it started with Jacob Spener. He saw a lack godly affection, or of godly passion for the practical application of doctrine, in the church. I won't argue that he was mistaken in this observation. He was probably right.
Where Spener went wrong was in his remedy for this lack. I believe the remedy was in the theological past. All that was needed was to revive old teachings, to remind the church of what she once knew and practiced.
Instead, Spener promoted - unintentionally, I suppose - a turning away from doctrine to practical Christianity, like loving people, feeding the poor, etc.. This is not bad, but when doctrine takes a second place to anything else, we go astray. Putting doctrine first is the best way to put Christ first. He is His doctrine. He said so Himself.
Second, Spener's ideas influenced John and Charles Wesley. They had an adversarial relationship with George Whitefield. They often refused to listen to him, and often preached against his preaching. Whitefield once left John in charge of his flock, and while he was gone, John countered his ministry from the pulpit. Dallimore recounts this in his book on Charles Wesley, which I've read.
And third, the Wesley's Methodism led straight into the Pentecostal revivals of the late 19th and early 20th Centuries. This is where Spener's slight errors of trajectory led to the church being miles off course.
I have been in a Pentecostal/full gospel church for 30 years, so I have experienced this first hand. My conclusion of those 30 years is that I resolutely deny that we are tripartite - spirit, soul and body, and I deny that we have two parts, and mind and a heart, and I deny that the intellect is the natural mind, as opposed to the spiritual mind, whatever that is. The heart is the intellect. It is the heart that thinks. They are the same thing. And we are two parts - soul and body. Or spirit and body, if you prefer. Soul and spirit are synonyms.
Take away those errors, and what is left of Pentecostalism?
So, where did the church first go wrong? With Jacob Spener. Or maybe it was James Arminius. But I don't want to get into that.