It is difficult to overstate the importance of Martin Luther. He began as a Catholic monk protesting abuses in the secularized church of 1500. He ended up, unwittingly, as the leader of the Protestant Reformation, which resulted in innumerable varieties of churches.
Along the way, he did so much more. He translated the Bible into German, shaping the language of the German people for the next 400 years. He wrote sermons, taught at the university, wrote a liturgy for public worship, wrote a catechism to teach children, and wrote a hymnbook.
He did the work of four or five men.
He fearlessly opposed the corrupt papacy of his day. (Popes then were more like secular kings, preoccupied with money, building and conquest.)
His greatest theological legacy came from the Book of Romans, and the return to the New Testament teaching on justification by faith.
Along the way he opposed other unbiblical teachings in the church of his day, including indulgences, the clergy-laity distinction, the mass, celibacy for priests, and more.
Luther struggled with depression and doubts. If he lived today, he would be diagnosed with OCD. But he felt God used these painful struggles in his life.
Because he became such a public figure, Luther came under heavy criticism, and at times he deserved it. But he was also misunderstood at times. He never let criticism derail him from the battle.
Luther loved the Bible. He was a bold man who feared God alone. He loved his Lord. God used him to reform the Catholic Church, to launch the Protestant Church, to give the
German people a much-loved Bible, and through him returned people throughout Europe to the Bible and biblical faith.
It could well be that Martin Luther is the most influential Christian leader since the New Testament was written.