For almost 800 years Magna Carta has been cited in arguments between free men. The ideas which make us who we are (and by us, I don't just mean the English, but the whole English-speaking world) began on the day King John, notorious from the Robin Hood legend, reluctantly signed.
DK is right to celebrate the anniversary, but makes the common mistake of saying that the Great Charter established the Great Writ, habeas corpus. In fact, it makes no mention of it. Blackstone cites the first recorded use of the Great Writ in 1305, but similar writs were issued earlier. Habeas corpus is a keystone of our freedoms, but Magna Carta did not set it in its place.
For me, Article 39 of Magna Carta is the most important;
This established an Englishman's right to "trial by his peers." For a commoner, this meant trial by jury, which has done more to keep us free than any other invention of our fertile civilisation. When it has gone, and we may be apprehended, charged and judged by paid employees of the state, English civilisation will have ended. Every man, however sophisticated his argument, who proposes a limitation of jury trial is King John's spiritual heir and therefore your enemy.
You may see Magna Carta online here and (significantly) here, where it is described as part of the "ancestry" of the US Constitution. You may read an English translation (the original was of course in Latin) here. You will relish such articles as
but you will be disappointed by most of the others. Much of the document deals with the banal interests of the barons who forced King John to Runnymede long ago. Some of the language (for example about the Jews) is offensive to modern ears. That's why it is vital to understand that it is not what this document says that matters. It is what it did. For the first time in history, it limited the power of the state. It ended the rule of men and began the rule of law.
King John's subjects, even most of his barons, believed him God's choice of ruler. Yet Magna Carta set boundaries to his power. Few have read its words (and most would be disappointed by them) but that is why Englishmen cite the Great Charter whenever they feel threatened. That is why any man who does not revere it is no Englishman at all. The legend of the Great Charter is more important than the fact. It is the Robin Hood of legal history, while the state is - perennially - the Sheriff of Nottingham.
Magna Carta remains a great litmus test. Modern enemies of freedom have this in common with King John; they believe in the essential benevolence of the state and feel no need for it to be limited. Like him, they will be restrained only by force. Like him, they are keen to weaken any power but state power. Now, as then, the enemies of freedom may be detected by their desire to seize or imprison men without the lawful judgement of their peers. They may be detected by their desire to limit or abolish jury trial.
Almost 800 years after it was signed, it is vital that Englishmen who wish to remain free understand not what the Great Charter says, but why it matters. England will be England only for so long as they do.