Martin Luther (d. 1546) and John Calvin (d. 1564) -- why are their beliefs and practices so different from their modern adherents?
What's remarkable is that big differences showed up even by the time of the First Great Awakening (mid-18th century), represented by the preaching of Jonathan Edwards and George Whitefield.
One of the most knowledgeable students of these matters is David Anders, a longtime Calvinist who ended up converting to Catholicism.
From an essay of Anders about his deepening study of Calvin's theology:
"Calvin shocked me by rejecting key elements of my Evangelical tradition. Born-again spirituality, private interpretation of Scripture, a broad-minded approach to denominations – Calvin opposed them all. I discovered that his concerns were vastly different, more institutional, even more Catholic. Although he rejected the authority of Rome, there were things about the Catholic faith he never thought about leaving. He took for granted that the Church should have an interpretive authority, a sacramental liturgy and a single, unified faith."
[The most interesting segment is from the 9:00 mark to 27:00].
"Calvin believed that the Eucharist provides an undoubted assurance of eternal life.
Resembling the Roman Catholic view, Calvin stated that the sacrament of the Eucharist provided the 'undoubted assurance of eternal life to our minds, but also secures the immortality of our flesh.' "
"Luther made an attempt to remove the books of Hebrews, James, Jude and Revelation from the biblical canon.
He did so because he believed these books went against certain Protestant doctrines.
"Luther believed that human free will didn’t exist. He wrote:
'With regard to God, and in all that bears on salvation or damnation, (man) has no free-will, but is a captive, prisoner and bondslave, either to the will of God, or to the will of Satan...
We do everything of necessity, and nothing by free-will; for the power of free-will is nil...' "