Islam is a monotheistic religion articulated by the Quran and teachings of Muhammad. It has over 1.6 billion followers globally, making it the second largest religion. The majority of Muslims are Sunni but there is also a Shia minority. Key aspects of Islam include five daily prayers, almsgiving, fasting during Ramadan, and pilgrimage to Mecca if possible in a lifetime. Islamic law covers both religious and legal matters for Muslims. Criticism of Islam has existed since its early formation regarding issues like the morality of Muhammad and authenticity of the Quran.