Why are Luther's 95 Theses seen as so important?

You have two questions in one - the title and the last sentence. I am answering to the last one.

There were some people before, who tried to explain to the Catholic Church that the greed is bad and to remain in it. Gus, Wycliffe.

But what was hard in 14th century, at Wycliffe's life, became impossible in 15'th. Wycliffe was not killed or expelled. Gus was burnt and declared to be an heretic. It was not real at Luther's time to try to make the Church be honest and to remain in it. Or alive. The only way to change the Church these times was from outside. And eventually, the threat to lose all influence to Protestants, made the Catholic Church to become less rigid and to change.

There is absolutely nothing strange in it.

Martin Luther wasn't against "Indulgences." He was against the "Sale of Indulgences".

It was not "expressed," only "implied," that if "the Bible is the central religious authority and that humans may reach salvation only by their faith and not by their deeds," the church should not be "selling indulgences." Because those "indulgences" were tokens of "good deeds" and not of "faith."

If so, by selling "indulgences" the Catholic Church was selling something it had no title to, in order to trick people into donating money. Then the Catholic Church was a seriously corrupted organization that needed to be replaced by something better.

There is another story behind this, which demonstrated in 1517 what most might think is a modern phenomenom.

The 95 theses were written in 1517. Supposedly, Luther didn't nail them to the church door, he posted them on a bulletin board outside of the church, and sent copies to the church leaders. These comments were directed at Johann Tetzel, a Dominican friar who was selling indulgences to help fund Pope Leo X's pet project: rebuilding St Peter's Basilica in Rome. As the resident theologian, Luther's original intent appears to have been to generate a scholarly discussion amongst church elders for what he saw was the commercialization of faith.

Indulgences, in that time frame, were a written document that gave forgiveness for sins committed, or forgiveness for sins that might be committed in the future, in return for a monetary contribution. Originally, they were hand written, in a time when not many people had the ability to write and those that could were kept very busy, so they were fairly rare, likely reserved for a few wealthy people. With the written indulgence, you didn't have to beg for forgiveness, you just turned it in and were forgiven.

But, note the date - 1517. Something else had happened, about 60 years earlier - Gutenberg had created the first printing press with moveable, reusable type. As a direct result, both the time required to produce a document, and the cost of producing that document, plummeted.

One of the first heavy users of the printing press was the Catholic Church, who was employing legions of scribes to copy scripture and other church documents. And, yes, indulgences went from something that was fairly rare due to the high cost of having it written, to something that could be turned out in the hundreds of copies, for little more than the cost of the paper. By 1517, the printing presses were cranking out indulgences as quickly as the presses could operate. Now, they could be produced at such a rate as to be salable to anyone, not just a select wealthy few, and friar Tetzel was marketing them to everyone.

That's when indulgences began to get out of hand... the church slipped into using them as a money generating tool, because they were now so easy to create. This is what Luther objected to - that the church was generating profit from what should be faith.

However, the story doesn't end there, and in the remaining tale can be found the other great impact of the 95 theses.

Luther not only sent copies to the church leaders, he also gave out copies to friends. Some of those friends had access to... a printing press. And, yes, they had copies printed up. Quite a few copies, as it turned out.

This Economist article describes what happened next.

Although they were written in Latin, the “95 Theses” caused an immediate stir, first within academic circles in Wittenberg and then farther afield. In December 1517 printed editions of the theses, in the form of pamphlets and broadsheets, appeared simultaneously in Leipzig, Nuremberg and Basel, paid for by Luther's friends to whom he had sent copies. German translations, which could be read by a wider public than Latin-speaking academics and clergy, soon followed and quickly spread throughout the German-speaking lands. Luther's friend Friedrich Myconius later wrote that “hardly 14 days had passed when these propositions were known throughout Germany and within four weeks almost all of Christendom was familiar with them.”

Thus, Luther experienced in 1517, what we think of today as 'going viral'.. an individual's words or creations resonating with a large number of people, because they could be communicated to a lot of people.

Luther's 95 theses are not only important for what they said, they are important in showing the power of mass communications. Luther wasn't the first person to raise these issues, but he was the first one to have his thoughts communicated en masse... courtesy of the printing press. With printed copies all over Europe, he couldn't be silenced, or burned at the stake as a heretic without serious repercussions for the church. Too many people had read the theses, and agreed with them.

Thus, began the first full fledged public relations war. Seeing the power of mass communcations, Luther embraced it wholeheartedly, and invented the pamphlet, a short document written in German, not Latin, so locals could understand it, and being a single folded sheet, it could be produced in a couple of days, when publishing a printed book could take over a month.

The unintentional but rapid spread of the “95 Theses” alerted Luther to the way in which media passed from one person to another could quickly reach a wide audience. “They are printed and circulated far beyond my expectation,” he wrote in March 1518 to a publisher in Nuremberg who had published a German translation of the theses. But writing in scholarly Latin and then translating it into German was not the best way to address the wider public. Luther wrote that he “should have spoken far differently and more distinctly had I known what was going to happen.” For the publication later that month of his “Sermon on Indulgences and Grace”, he switched to German, avoiding regional vocabulary to ensure that his words were intelligible from the Rhineland to Saxony. The pamphlet, an instant hit, is regarded by many as the true starting point of the Reformation.

Luther also became a prodigious author. The church responded by holding the first organized book burnings... of his published works.

It is true that simply printing the 95 theses didn't bring about the Reformation... but Luther's words, because they resonated with so many of the general population, became a rallying point. The situation went from 'I don't really like the church' to 'let's get rid of the church and start our own'.

For another take on this most interesting story, review the 'A Matter of Fact' episode of the James Burke series The Day the Universe Changed.