Answer:
In my opinion it would be the Bubonic Plague.
Step-by-step explanation:
The Bubonic Plague that struck Europe in the middle of 14th Century devastated European population. People lost their faith in the role of certain institutions, especially Church and realized that the Church cannot protect them. As many died, need for peasants who were work on the manors increased, but now the peasants had the opportunity to claim certain rights. Additionally, people realized they need to change their way of thinking and functioning and they were to prevent these things to happen.