The timeframe was early in the 16th century. The Roman Catholic Church was pretty much the Christian church. Most of its competition (e.g., the churches in Alexandria and Antioch) had been swallowed up by Islam hundreds of years earlier; this process was finally completed with the fall of the Byzantine Empire to the Ottoman Turks in the mid-15th century. (The Eastern Orthodox Church today is a remnant of the Byzantine Church.) The Roman church was also significant as a political power -- there was no "separation of church and state" in 16th century Europe. Politics is corrupting, and corrupt people often decide that a practice is ethical just by checking to see whether it is productive of revenue.
Two practices that were very productive of revenue for the Church were the selling of relics, and the selling of indulgences. The relics were items claimed by religious hucksters to have special significance with respect to Christ or his disciples -- e.g., pieces of his cross, bones from a disciple, and so forth. The indulgences were "guarantees" from the Church of forgiveness for a particular sin, whether for oneself or for a loved one suffering in Purgatory -- "As soon as a coin in the coffer rings, a soul from Purgatory springs."
Even more disturbing, however, was the shameful suppression of the Bible by the Church hierarchy. At various times in Church history, it has been against the law, and punishable by death, to possess a Bible, or one that had been translated into one's own language. It would appear that the Church wanted a monopoly on theological thought, and would dispense the Lord's wisdom, or their own wisdom relabeled using the Lord's brand name, as they saw fit.
A handful of men led the rebellion against such corruption. Martin Luther, Ulrich Zwingli, and John Calvin changed the world profoundly, and not merely in theological terms. Until the Reformation, the centers of Western Civilization had always been Italy, Spain, and France; soon, they were to be rivaled by Protestant Holland, Switzerland, various German states, and the emerging economic dynamo that was England. In these countries, the Bible replaced the Pope as the ultimate religious authority. Martin Luther's German translation of the Bible, followed by the Geneva Bible, and (soon after that) the King James Bible put the word of God directly into the hands of Christians, and in their own language. We see the Bible today everywhere we go, even in hotel rooms (thanks to the Gideons), and we take that for granted today, but it took about fifteen centuries, and a lot of martyrs, to make that happen.
It was a tragedy, of course, that Christianity became split, and we take it on faith that someday we shall be united again. However, relics are meaningless, and the Lord alone can forgive sin -- and He can do so without the help of a middleman. Originally a response to corruption within the Church, the Reformation provided a much-needed reinvigoration of theology simply by restoring the authority of God's own word. Ideas have consequences, to borrow Richard Weaver's signature phrase, and this reinvigoration played a huge role in shaping the world we know today.