At noon on October 31, 1517, a hammer struck the door of the Castle Church in Wittenberg, Germany. It was the start of the Reformation.
The Ninety-Five Theses, composed originally in Latin, were posted by Martin Luther on the door of the Castle Church in Wittenberg, Germany, on October 31, 1517. The Castle Church was used by the university as its “campus church” and as such the door served as a sort of public bulletin board for the academic community. The theses were a proposal for a discussion about the practice of indulgences. October 31, 1517, the day before All Saints Day, was chosen because the Castle Church was also home to one of the largest collection of relics in all of Western Christendom, owned by the Saxon Elector Frederick the Wise. Indulgences were granted to the faithful for viewing the many relics that were put on display on All Saints Day. 
When Dr. Martin Luther, a law student turned Augustinian monk turned great Reformer, nailed his 95 Theses to the church door, little did he know that it would be copied, translated, and distributed throughout all of Europe. Little did he know that his name would be recorded as one of the most significant figures in church history.
The fact that Martin Luther is an identifiable character in the story of human existence is secondary to what was divinely accomplished through his scrutiny of unbiblical religious practices. As an instrument in God’s Hands, he would spend the remainder of his life reintroducing the largely forgotten truths of the Christian faith.
Luther’s diligent study and subsequent teaching of the Bible has led to many reforms we might otherwise take for granted today. He refuted the sale of indulgences as a means of salvation, turning instead to the New Testament teaching of salvation by grace alone, through faith alone—and justified through Christ and His righteousness alone. 
Luther’s translation of the Bible into German vernacular put the Word of God in the hands of the common people. He further translated the liturgy (i.e. the worship service, which was traditionally in Latin) into the common language so that all could understand it.
Luther also challenged the marriage ban placed on clergy. He showed it to be unbiblical and, by his own example of marriage, dramatically reformed the vocation. Not only that, he reintroduced the biblical view that all Christians—not just the clergy—serve their Creator and their neighbors through their honest and faithful work, whatever it may be.
Today, Luther’s contributions to the Christian faith are seen in the creeds and confessions of Protestant congregations around the world. But make no mistake. Luther did not reinvent the Christian faith. Nor did he reinterpret it. Luther simply rediscovered the one true faith that was once delivered to the Saints.
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Lord, keep us steadfast in Thy Word;
Curb those who fain by craft and sword
Would wrest the kingdom from Thy Son
And set at naught all He hath done.
Lord Jesus Christ, Thy pow’r make known,
For Thou art Lord of lords alone;
Defend Thy Christendom that we
May evermore sing praise to Thee.
O Comforter of priceless worth,
Send peace and unity on earth.
Support us in our final strife
And lead us out of death to life.
— A hymn by Martin Luther, 1541