memes

The widespread idea that peasants could not read is something of a falsehood. While certainly they were not considered "literate" by census at the time that is in part because literate had a different bar to meet. If you could only write whatever language you spoke as daily vernacular and hadn't gone through a six year set of schooling with a specific reading list (known as completing your letters) you were considered illiterate by the measure of the time. It is true that peasants rarely could afford to become "literate" by this definition so if someone says only the clergy and nobility were literate technically speaking they are correct.

However.

Archeological evidence posits that in medieval Europe writing vernacular was a fairly widespread and vital skill though most surviving examples of peasant writing were on birch bark and were missives under 20 words in length. There is evidence that both men and women demonstrated and used the skill primarily for placing orders, sending invitiations, IOUs, sending personal news and messages. The skill was widespread enough that peasants in England and France were written to by clergy and nobility as audiences with things like manuals for peasant farmers and housewives to read.

Sadly because reading vernacular didn't count as a skill unique enough to note in medieval census reporting we have to guess at how much of the population actually could read and only know the skill spread and became more common with time.