Final answer:
The Church started to become 'worldly' during the late 1300s and 1400s, characterized by internal power struggles, corruption, and the Pope's involvement in political affairs. Martin Luther's actions in the early 1500s challenged and successfully reformed the Church's practices.
Step-by-step explanation:
The Church started to become 'worldly' during the late 1300s and 1400s, when it was plagued by internal power struggles and corruption. Popes and Cardinals lived more like Kings, claiming political as well as spiritual power. They commanded armies, made political alliances, and even waged war. The corruption and abuse of power within the Church were well-known, and several attempts were made to reform it. However, it wasn't until Martin Luther's actions in the early 1500s that the Church's practices were successfully challenged.