hey Bro...
you said “that didn’t really begin until Gutenberg’s Bibles, circa 1450.”
however, you left out that Gutenberg was Catholic and printed ...horrors...Catholic Bibles.
Lurking’
Circa 1450 there were no such things as "Protestants".
There was no Martin Luther, John Calvin, Ulrich Zwingli, Henry VIII or Scottish Presbyterians.
In 1450, most of those were still twinkles in their grandfathers' eyes. ;-)
In 1450 Luther's Bible translation was still nearly a century away, and the English King James Bible still 150 years in the future.
And Gutenberg's Bibles were in Latin, so it still took highly educated people to read them.
Nevertheless, it was a start: Gutenberg printed hundreds of Bibles, and other printers soon produced thousands more, essentially flooding the market for anyone who had the ability and desire to read them.
And many, many people did, and it opened their eyes, and they began to ask questions, embarrassing questions, questions the Roman Church could not always answer to their satisfaction.
And so, slowly, slowly, with fits and false starts, the Protestant spirit was born and eventually captured the imaginations of millions.
The rest, as they say, is history...