Principle number 9 of Basic Elements of Islam: You can't leave Islam once you're in. This is an interesting one. It is actually illegal in Islamic states to convert out of Islam. This is a critical part of Sharia law. Someone who has rejected Islam who was once a Muslim is an "apostate." This is a crime and a sin, and the punishment for it is death (and eternal damnation in hell thereafter).
Obviously, you can see why this idea has been included in the collection, but this one has actually caused Islam a problem because those who are following Islam to the letter consider more "moderate" Muslims (those who want to ignore or alter the more violent passages of the Quran) to be apostates. Since the punishment for apostates is death, fundamentalist Muslims are fighting modernizing Muslims all over the world, and keeping many rebellious, modernizing Muslims from speaking up for fear of death.
Every time a group of Muslims decides that maybe Islam should be updated for the 21st century and maybe women should have some rights and maybe the government should be more democratic, the devout Muslims call them apostates and discredit them or even try to kill them.
In this and in many other ways, Islam protects its own fidelity (in other words, the original idea-collection cannot be altered).
Another idea in Sharia law says it's against the law for anyone to try to convert a Muslim to another religion.