Answer:
Because it showed that change is possible.
Explanation:
The Protestant Reformation of the sixteenth century was a period of huge change and development not only in the Christian world but also worldwide. This reformation led to a massive surge in the prospect of change both in the religious and political scene.
Before the Protestant Reformation, there was only one church that ruled over all of Europe, which is the Roman Catholic Church led by the Pope. This Church made rules and would be the sole form of religious belief that lead the people. But with the increasing want of lesser papacy control over both the religious and political affairs, added with the issue of corruption within the church itself, rulers of different countries began to press for a separation of the Church and the state. This means that the church, (Pope) will no longer hold power over political affairs.
So, when Philip Schaff, a Protestant theologian stated that the Reformation is one of the<em> "greatest event(s) in history"</em>, he is expressing his belief that Protestantism is responsible for showing that change is indeed possible even at times of larger Catholic hold over the nation. He went on to express his belief that it gave <em>"a mighty impulse to every forward movement, and [became the] chief propelling force in the history of modern civilization."</em>
