Islam is an Abrahamic monotheistic religion. Its core teaching is that there is only one God, and that Muhammad is the messenger of God. It is the world's second-largest religion, with over 1.8 billion followers or 24.1% of the global population. Followers of Islam are known as Muslims, and comprise a majority of the population in 50 countries. Islam teaches that God is merciful, all-powerful, unique and has guided mankind through prophets, revealed scriptures and natural signs. The primary scriptures of Islam are contained in the Quran, which is perceived by Muslims to be the verbatim word of God. Muslims divide into two groups, Sunni and Shia, with the separation dating from the death of the Prophet Muhammad. The Sunni majority believed that his successor should be his father-in-law, Abu Bakr, but a smaller Shia group considered that the Prophet Muhammad’s successor should be Ali ibn Abi Talib, his cousin and son-in-law, and father of his grand-children.