Jihad.

One of the worlds most popular religions is Islam. Founded by Muhammad around 620 A.D., Islam contains ideas, rules and concepts different than most. These concepts include the hajj, the Hegira and jihad.
Jihad is literally defined as "struggle in the way of God". This word can sometimes be translated as "holy war". This principle basically says that Muslims are to fight against all people who use idolatry and do not worship Allah. Daniel Pipes wrote an article on jihad for New York Post saying, "It means the legal, compulsory, communal effort to expand the territories ruled by Muslims at the expense of territories ruled by non-Muslims." (source)
The jihad grew out of the Arabic tradition of tribal rampages. Islamic followers would raid certain regions and countries in the name of Allah. These raids went on for over 100 years. It was finally stopped n Tours, France. They would raid these regions attempting convert anyone from another religion to convert to the Islamic religion.
Most Muslims believe that jihad is an everlasting war. They believe that it never ends. They may not be physically fighting for their religion, but they in a religious war to convert other religions to worshiping Allah. The jihad will continue until the end of existence, they believe.
Islam, being the number one religion in the world, is mainly this widespread because of jihad. They forced their ways and converted thousands upon thousands of people to the way of Allah. The jihad is not over, and I personally believe that as time goes on, the Islamic will only get pushed even more and become more widespread through the world.