key contributions. When, following the revelation of the surah at-Tawbah, Abu Bakr was sent to Mecca to give an ultimatum to disbelievers, there is strong evidence that Muhammad might have sent out Ali to take over this responsibility. Ali's role In Shia theology, while direct revelation ended with Muhammad's death, Ali remained the righteous guide towards God, similar to the successors of the past prophets in the Quran. After Muhammad's death, Ali inherited his divine knowledge and his authority to correctly interpret the Quran, especially its allegorical and metaphorical verses (mutashabihat). Often cited here is a well-attested hadith, attributed to Muhammad, which reads as, "I am the city of knowledge and Ali is its gate." Ali's infallibility In Shia Islam, as the righteous guide after Muhammad, Ali is believed to be infallible. Ali is one of the Ahl al-Kisa, who are addressed by the sahih Hadith of Kisa and the related Verse of Purification in the Quran, which includes the passage, "Indeed God desires to repel all impurity from you, O Ahl al-Bayt, and purify you with a thorough purification." Imamate According to the Shia, Ali succeeded Muhammad as the first Imam after Muhammad, that is, the righteous guide towards God and His vicar on the earth. This divine authority, known as imamate, is central to the Shia belief and appears in multiple verses of the Quran. In particular, verse 21:73 reads as In the Twelver Shia belief, since the time of the first prophet, Adam, the earth has never remained without an Imam, in the form of prophets and their divinely-appointed successors. After Ali, imamate was passed down to his son, Hasan, through divinely-inspired designation (nass). In Shia theology, at any time, there is only one Imam and his successor, if alive, is called the silent Imam. After Hasan's death, his brother, Husayn, and nine of his descendants are regarded as Imams, the last of whom, Mahdi, went into occultation in 260 AH (874 CE), compelled by the hostility of his enemies. His advent is awaited by all Muslims, though different sects hold different views about Mahdi. In his absence, the vacuum in the Shia leadership is partly filled by marjaiyya and, more recently, wilayat al-faqqih, i.e., guardianship of the Islamic jurist. Zaydi According to Jafri, it is widely reported that the fourth Shia Imam, Zayn al-Abidin, designated his son, Muhammad al-Baqir, as the next Imam before his death.