Sunni Islam is the largest branch of Islam and the largest religious denomination in the world. Shia Islam is the second-largest branch of Islam.
References
Axworthy, M. (2017, August 25). Islam’s great schism. New Statesman, 146(5381), 22–27. https://www.newstatesman.com/world/middle-east/2017/08/sunni-vs-shia-roots-islam-s-civil-war
Louër, L. (2014). The State and Sectarian Identities in the Persian Gulf Monarchies: Bahrain, Saudi Arabia, and Kuwait in Comparative Perspective. In L. G. Potter (Ed.), Sectarian Politics in the Persian Gulf (pp. 117–143). Oxford University Press.
Jones, J. (2016). Sectarian Gulf: Bahrain, Saudi Arabia and the Arab Spring That Wasn’t [Book Review] Journal of Islamic studies, 27(2), 242–243. https://doi.org/10.1093/jis/etv108
Matthiesen, T. (2013). Sectarian Gulf: Bahrain, Saudi Arabia, and the Arab Spring That Wasn’t. Stanford University Press.
Potter, L. G. (Ed.) (2014). Sectarian Politics in the Persian Gulf. Oxford University Press.