For me, the divide between the Middle Ages and the Renaissance has long been the invention of the printing press by Gutenberg.
With the invention of the printing press, documents began to be much more widely available.
The impact of this was that the attitude towards the Church shifted dramatically from the Middle Ages to the Renaissance as people began getting access to more documents and used those documents to think critically about the Catholic Church and its practices.
So, by the time of Luther, there was a fervent group of informed and educated reformers who were influenced by Renaissance views on critical thinking who took issue with the sale of indulgences in a way that was not done during the Middle Ages.