When I teach the apostasy and restoration aspects of the Melchizedek Priesthood prep class, I focus on some of the foundation Gospel principles of the restoration that were direct refutations of the current religious beliefs that dominated the Protestant Reformation.
The most fundamental example of a challenge to the very foundation of the Reformation was Joseph’s insistence that, although Catholicism had screwed it up over the years, divine authority actually was meant to reside with mankind. Protestant reformers knew that they did not have “divine authority” (and said so explicitly), and they recognized clearly that the Catholic Church had lost that authority. Their only “logical” conclusion, given the lack of divine authority vested in men, was to shift such authority to “the Word of God” - since they knew they couldn’t claim it for themselves. That “Word of God” was the Bible - all that they had that everyone agreed represented His words to man - and that investiture almost required that inerrancy become the standard, since any errors would destroy all sense of God’s authority existing at all on the earth. Finally, if the authority of God was invested in the Word of God, the only authority available to man was through a deeper and fuller understanding of that Word - which opened the door for Divinity Schools and Masters of Theology degrees - and their lay counterpart, the individual preacher who could quote the Word in a charismatic and inspiring way.
Joseph Smith shook the very foundation of their claims to divine authority - not just by claiming to have been called as a prophet, thus restoring divine authority to man, but also by introducing an additional source of God’s word (The Book of Mormon), thus removing the exclusivity of the Bible. In essence, he said, “Well, you did the best you could with what you had, but the old system you rejected is back in place with new materials - so accept a system your very foundation rejected or run along and play your human games.”
Many members have a basic understanding of this conflict, but I think not nearly enough really understand why the conflict produces such intense and vitriolic reactions. Given what Joseph taught, and what it means about the very core creeds of Protestantism, the reaction doesn’t surprise me one bit - then or now.