This day, 500 years ago, is generally considering to be the start of the Protestant Reformation. Although the wheels of history were already turning, it was on this momentous day that Martin Luther nailed his 95 Theses to the church door in Wittenburg, thus beginning his long battle to reform the church.
Much of what we believe as a church, we owe to him and the reformers who came after him. The Lord used His people to turn away from the powers of the Earth to return to the Scriptures themselves. All of Protestantism can be traced back to this moment in history.