Islam is an Abrahamic monotheistic religion centered on the Quran and the teachings of Muhammad, the religion's founder. Adherents of Islam are called Muslims, who are estimated to number approximately 1.9 billion worldwide and are the world's second-largest religious population after Christians.
Muslims believe that Islam is the complete and universal version of a primordial faith that was revealed many times through earlier prophets and messengers, including Adam, Noah, Abraham, Moses, and Jesus. Muslims consider the Quran to be the verbatim word of God and the unaltered, final revelation. Alongside the Quran, Muslims also believe in previous revelations, such as the Tawrat (the Torah), the Zabur (Psalms), and the Injil (Gospel). They believe that Muhammad is the main and final Islamic prophet, through whom the religion was completed. The teachings and normative examples of Muhammad, called the sunnah, documented in accounts called the hadith, provide a constitutional model for Muslims. Islam teaches that God (Allah) is one and incomparable. It states that there will be a "Final Judgment" wherein the righteous will be rewarded in paradise (Jannah) and the unrighteous will be punished in hell (Jahannam). The Five Pillars—considered obligatory acts of worship—comprise the Islamic oath and creed (shahada); daily prayers (salah); almsgiving (zakat); fasting (sawm) in the month of Ramadan; and a pilgrimage (hajj) to Mecca. Islamic law, Sharia, touches on virtually every aspect of life, from banking and finance and welfare to men's and women's roles and the environment. The two main religious festivals are Eid al-Fitr and Eid al-Adha. The three holiest sites in Islam are Masjid al-Haram in Mecca, Prophet's Mosque in Medina, and al-Aqsa Mosque in Jerusalem.
The religion of Islam originated in Mecca about 610 CE. Muslims believe this is when Muhammad began receiving revelation. By the time of his death, most of the Arabian Peninsula had converted to Islam. Muslim rule expanded outside Arabia under the Rashidun Caliphate and the subsequent Umayyad Caliphate ruled from the Iberian Peninsula to the Indus Valley. In the Islamic Golden Age, mostly during the reign of the Abbasid Caliphate, much of the Muslim world experienced scientific, economic, and cultural flourishing. The expansion of the Muslim world involved various states and caliphates as well as extensive trade and religious conversion because of Islamic missionary activities (dawah), and through conquests.
The two main Islamic branches are Sunni Islam (85–90%) and Shia Islam (10–15%). While the Shia–Sunni divide initially arose from disagreements over the succession to Muhammad, they grew to cover a broader dimension, both theologically and juridically. Muslims make up most of the population in 49 countries. Approximately 12% of the world's Muslims live in Indonesia, the most populous Muslim-majority country; 31% live in South Asia; 20% live in the Middle East–North Africa; and 15% live in sub-Saharan Africa. Muslim communities are also present in the Americas, China, and Europe. Largely due to having a high proportion of young people, and a high fertility rate, Muslims are the world's fastest-growing major religious group.