This theme will have a few parts to it. Here I just want to sketch out the logical/historical link between Protestantism and the abandonment of Christianity.
The essential claim of Christianity is the incarnation - that Jesus is 100% human and 100% divine. One of the consequences of that claim is that the material world, the flesh, can be a vessel for the sacred, that it can communicate the transcendent, that it can be a means of grace.
This is the foundation for a sacramental theology, ie that through the water used in baptism, through the bread and wine of communion, God actively engages with the faithful and works to their healing - here the signs and symbols employed mediate God's grace.
Consequently the historically orthodox churches have all emphasised the centrality of sacramental worship - baptism as the rite of entry into the church, communion as the central act of worship renewing and sustaining the church. This pattern is common across the vast majority of Christianity in time and space: Eastern Orthodox, Roman Catholic, Anglican, Lutheran etc. However, it is a pattern that broke down after the Reformation in some Protestant churches, and which became culturally influential.
In England, for example, the downplaying of the altar, and the raising up of the pulpit to a position of great prominence, can be tracked architecturally. Whereas the historic faith had been sacramentally centred, the post-Reformation church reduced the sacraments to simple signs - and beyond that, they were signs that were optional for faith. The essence of Christianity became "faith", as in being "justified by faith" - and this became reduced to a matter of right belief. If you believe that Jesus Christ is your personal saviour then you have a saving faith.
So, in a great many Protestant churches today, what is most central to Christian worship is right teaching. You have to have the right attitude to the Bible, and the teachers of the Bible have to teach the right thing. Those are the essential elements for ensuring salvation.
However, note what has been lost in this transition. Where the sacraments have become optional or redundant, and teaching takes its place, you no longer need Jesus to have been God incarnate. He just needs to have been a good teacher. Where salvation is a matter of right belief, then Jesus' prime purpose is to teach that right belief (despite the fact that Jesus never uses the phrase "justified by faith").
So in the countries dominated by Western Protestantism, where the sacraments were downplayed or ignored, the idea that Jesus was simply a good teacher was implicitly first taught within the churches themselves. Of course, a major corollary of this trajectory of Protestantism was that church itself eventually becomes redundant. For if salvation flows from right belief, and right belief is a matter of rightly understanding the Bible and what it teaches - then that is something that can be obtained by private reading, private study of the Bible. Which becomes: "you don't have to go to church to be a Christian", a refrain still commonly heard on English streets when talking to the vicar.
At this point - when the church is redundant, when Jesus is simply a good teacher - the mental effort required to move to rejecting Jesus as a principal teacher is not very far.
This brings us on to questions of God, which I'll cover in another post.