The primary Shia hadith book, al-Kulayni's Usul Al-Kafi, utilizes Jesus in several essential ways. One is to extend the Quran's own presentation of Jesus as a human prophet of God and a forerunner of the Prophet Muhammad. Jesus in the Quran and Al-Kafi specifically repudiates Christian theology, condemning the trinity as a polytheistic deviation from his true teachings. Al-Kulayni displays clear knowledge of the Bible, citing Biblical stories of Jesus that do not appear anywhere in the Quran. Al-Kulayni clearly includes these stories as a polemical attempt to convert Christians to Shia Islam, and to defend his faith from Christian criticisms. Furthermore, Jesus is also utilized to defend specifically Shia theories against Sunni criticism. Namely, Jesus's ascension to prophethood at a young age and speaking from the cradle were likened to the Twelve Imams. Many Sunni critics of Shia Islam claimed that the Shia Imams could not be the rightful leaders of the Muslim world based on their youth. Shia theologians responded by emphasizing the youthful nature of Jesus.