The religion of Twelver Shia Islam developed in a highly factionalized, turbulent, and sectarian time-period. The main Shia book of hadith, Usul al-Kafi, was written specifically to defend the doctrines of Twelver Shia Islam against the theological polemics of other religions, such as Judaism and Christianity, which competed with Islam for followers during the Islamic Middle Ages. Usul al-Kafi also contains multiple polemics against other Muslim groups, most notably Sunnis and Zaydis. The powerful and influential Ismaili sect, which had split with the Twelver sect by the time Usul al-Kafi was written, is also targeted by Al-Kulayni, the compiler of Usul al-Kafi. Ismailism and Twelver Shi’ism dispute the succession to Jafar al-Sadiq, whom they both recognize as a rightful Imam of God. Ismailis maintain that the Imamate passed from Jafar onto his eldest son Ismail, the recognized heir apparent during Jafar’s lifetime, then onto Ismail ibn Jafar’s son, Muhammad ibn Ismail. The Ismailis continued to recognize successors to Muhammad ibn Ismail in a line which continues until this day. The Twelver Shia on the other hand, recognized the succession of Imam Jafar’s younger son, Musa ibn Jafar, and a select number of his descendants which ended with the twelfth Imam, who is believed to exist in a state of occultation. Ismailis wrote detailed polemics against the Twelvers, defending the succession of Ismail ibn Jafar, and scoffing at the idea of a hidden Imam. Al-Kafi contains multiple hadiths which defend the succession of Musa ibn Jafar as the rightful Imam, clearly a defensive polemic against Ismailism. Furthermore, Al-Kafi contains an extensive elaboration on the concept of al-Bada. Al-Bada is best described as an apparent change in God’s will. Al-Kafi utilizes the concept of al-Bada prolifically to defend the selection of Musa ibn Jafar, against Imam Jafar’s widely recognized heir apparent. A thorough study of the polemical nature of Al-Kafi, specifically with regards to Ismailism, can shed light onto the formative period of Twelver Shi’ism. Specifically, it can help scholars better understand the highly sectarian time-period in which modern day Muslim sects developed. It will also shed light on the creative imagination of Al-Kulayni, one of the most influential thinkers in Shia history.