Today, focusing on Shia-Sunni relations has become an inevitable necessity to understand and make sense of many developments in the Islamic world. Many things perceived as new and called modern developments are, in fact, merely history repeating itself in a different form. The book in your hands addresses a concise religious/political history of Shiism, the process of its deviation from mainstream Islam (Ahl al-Sunnah wa'l-Jama'ah), its fundamental claims and theses, and the responses of Sunni scholars. The study views Ahl al-Sunnah wa'l-Jama'ah as the main path and highway of Islam, rather than considering it an independent sect. It defines the Shia as one of the factions that has diverged from this main road through ambiguous messages, distinct discourses, and far-fetched interpretations. According to this thesis, Islam has continued its existence uninterruptedly through the path of Ahl al-Sunnah, alongside the fundamental message of the Quran and the Prophethood, through the representation and transmission of all the Prophet's close companions and the overwhelming majority of the Islamic community (the jama'ah/mass majority).