See:
The Great Apostasy Develops — Watchtower ONLINE LIBRARY a good explanation of the apostasy that developed in the Christian Church after Jesus' death.
Here is the chapter in Edward Gibbon's voluminous work that goes into some detail about this falling away and apostasy of the truth:
https://erenow.net/ancient/fallromeempire/volumei/26.php
The idea that Jesus was god was not taught by his original apostles or followers. But in the succeeding centuries after his death many sects of Christianity developed, which eventually lead to the Athanasian Creed.
The Protestant Reformation was a protest against many of the abuses of the Catholic Church and the schism was further driven among the warring sects of Christendom. But the reformations did not get rid of many of the pagan teachings that had seeped into the Church and corrupted Christianity over the centuries. It was not God's time.
Notice that it would be during the time of the end that the wheat would be separated from the weeds. The weeds appear as wheat, they are imitation Christians.
Jehovah's Witnesses are not a sect of Christendom as are the many branches of Protestants.
Rather they are a return to the original form, or true Christianity. This is a work of Jehovah God and not that of any man.
Remember the illustration of the wheat and the weeds.
This right here is the heart of the matter. The JW's separate themselves from what they term "Christendom" as a reaction to a poorly-understood "interpretation" of Matthew. It was never Matthew's intention to cause division within the Church. Matthew's inclusion of the parable isn't concerning either doctrine or theology. The Protestants, by and large, do not distance themselves from the continuity of the Faith; they still revere the saints, many ascribe to the Apostolic Succession, and they revere the church Fathers and Mothers. For the Protestants, there is no "Great Apostasy," merely somewhat of a departure on the part of the RCC (not sure where they stand on the issue of the Orthodox; their "beef" seems to be specifically with the RCC) from certain
points of doctrine that the Protestants feel are "unbiblical." Most Protestants are Trinitarian.
I feel that it's their declaration of this "Great Apostasy" that separates them (and invalidates their claim to Church heritage). If, as they say, some "Apostasy" occurred, this would make the JW's little more than a "mulligan" attempt (for those of you who play golf) at being the Church. They claim to "return to the 'original form,' or 'true Xy.'" However, history makes clear that there never really was one "original form" of Xy. Each church always had its own flavor of orthodoxy and orthopraxy -- even up to the present day. Rome has never agreed with Constantinople, just as the Church in Jerusalem didn't agree with Paul in his own time. The JWs attempt to "restore" Xy to some "true" or "authentic" form, but in so doing, they completely ignore all Apostles' teaching outside their unique interpretation of the Biblical texts (which has never been the practice -- even among the first "twelve," including Paul and their
immediate successors. I believe this faulty historical perspective is, in substance, a "conspiracy theory," which attempts to control or push an agenda, ultimately through fearmongering. It changes historical facts and rewrites a "new 'truth,'" in which only the theorists can provide the sure path to salvation. In this way, they are able to discredit the historic authorities as "liars" (their term is "apostates").
"True" Xy (if such a thing even exists in some definitive form) is marked by unity in diversity -- not uniformity in sameness. Most Protestants agree that other churches are valid; even the RCC acknowledges that other churches have validity in the eyes of God. Our differences are largely preferential and not ontological. But the JWs insist that only
they have an existence that is "real" in spiritual terms. This utterly dismisses the validity of every other church. It is a difference of ontology. [edit] As Exchemist says, it represents a "No True Scotsman" fallacy. Thanks, Exchemist!
The historic Church (including RCC, Orthodoxy, Anglicanism, and all Protestant bodies) is the body of Christ. Spiritually and theologically, that is our usual ontological understanding. Even among denominations that typically don't "like" each other. It is Biblical, and it is part of the Apostolic teaching since the church's inception. The JW stance denies the rest of us that ontological reality.
By doing this, the JWs are able to interpret the Biblical texts as they see fit, claim "authority" and deny the rest of us -- including what has traditionally been taught by Apostles and other authorities. I believe this is the root difference. All other differences in belief, practice, appearance, and doctrine stem from this root belief.