Principle number 17 of Basic Elements of Islam: Allah gives Himself permission to edit His own work. This is an interesting one. It says in the Quran that if a passage written later contradicts an earlier passage, then the later one is the better one (read more about that). The Quran was written in sections (each section is one of Mohammad's revelations, known as a sura or chapter) over a period of 23 years. The circumstances of Mohammad's life and his religion changed quite a bit over those 23 years.
One of the ideas in the Quran is "this is the word of Allah." People had already memorized his earlier revelations, and it would seem a little strange for the all-knowing, infinitely wise Allah to change something He had already said. But with this idea that later revelations abrogated or overwrote earlier revelations when they contradicted, the newer ideas could be accepted. Allah could edit His work.
As I pointed out earlier, in his first 13 years of peacefully preaching, Mohammad only managed to win 150 followers. But as a military leader and violent conqueror, he was able to subjugate all of Arabia to Islamic law in less than 10 years. The peaceful ways were slow. Conversion by conquering and establishing Sharia was faster and more efficient.
The bad news for non-Muslims is that the later, violent, intolerant verses abrogate the earlier peaceful, more tolerant passages. Read more about which passages were abrogated and what the last passages say.