The Diet of Worms
At the Diet of Worms in 1521, Martin Luther rebuked the Pope and the Holy Roman Emperor by saying, "Here I stand, I can do no other." Except he didn't exactly say that, and he had already rebuked both the Pope and the Emperor before 1521. But if Luther had not taken the actions he did at the Diet of Worms, the entire Protestant Reformation would maybe not have happened, or happened quite differently. Also, Martin Luther would have just become another John Wycliffe or Jan Hus, rather than the signature reformer. In other words, the Diet of Worms made the Protestant Reformation what it was, but everything could have been different.