The main reason why Islam is split into the two major factions that we know today is due to a succession crisis that occurred in Islam after the death of prophet Muhammad.
Essentially, Sunni Muslims believe that Abu Bakr as-Siddiq, prophet Muhammad’s father-in-law, is the rightful successor to become the first caliph (political and spiritual leader of the Muslim community) while Shia Muslim believe that Ali ibn Abi Thalib, prophet Muhammad’s cousin and son-in-law, should have been the first caliph instead. In the end, Abu Bakr as-Siddiq was elected as the first caliph, creating the first tension between the Sunni and Shia. Although eventually, Ali ibn Abi Thalib was elected as the fourth caliph, the divide continues to deepen until now with differences in religious customs and practices.