@Sunstone I would say the invention of the printing press
democratized the "
market-place" of Christian ideas.
By this, what I'm saying is that it offered an hitherto unparalleled means of quickly disseminating new theological concepts and radical beliefs, which meant that the Reformers of the 16th century had a communication method available to market and promote their theologies which preceding Christian sects (such as the medieval Lollards, Waldensians and Cathars) lacked.
This might be one of the reasons why the Protestant Reformation spread so quickly from Germany to France, Britain, Scandinavia and Switzerland etc. whereas earlier attempts at reform (with the exception of the Papal Reformation of the 12th century and the Hussites in the 15th century Bohemia) had failed.
I think the printing press opened the minds of readers to new possibilities and in this way 'loosened' the hold that orthodox Christianity and the established Roman church had over the faithful in a number of European countries. Particularly key to this were the early prints of the Bible in the Vernacular, such as William Tyndale's New Testament (1522) and Martin Luther's German Bible (1534). With books more freely available to the masses than ever before, every literate person could now hope to read Scripture for themselves, rather than mediated through clerics at the pulpit or theologians at universities.
A theological idea that was notably conducive to such an environment is the Protestant doctrine, preached by Luther and Calvin, of the "
priesthood of all the faithful" which emphasised that no intermediary (i.e. a cleric) was needed between an individual and God but that all believers were in some sense equal before God. This led to the formation of a less hierarchical model of church structure in Calvinism, with Presbyters / ministers rather than bishops and priests, and the even more radical ecclesiology of Baptists and other congregationalist churches which abolished pretty much all clerical titles and roles, rendering all Christians laity.
On the negative side of the equation, it also enabled the Protestant Reformers to spread sometimes woefully inaccurate portrayals or caricatures of Catholic doctrine which have had a pernicious influence till this day.