The protection provided by twelve Caliphs
Shiites – and also Sunnis – believe that the prophet has ensured the survival and dignity of Islam until the Judgment Day through the protection provided by twelve Caliphs. Sunnis have been incapable of introducing these twelve Caliphs; however, for Shiites, these twelve divine Caliphs are identified. Through studying the history of their lives, it is revealed that unfortunately, "oppressive regimes’ pressure" caused "the birth of the last divine caliph" to happen in secret, like that of Abraham and Moses. This form of birth, created doubts; therefore, most of the Sunnis deny his birth. Shia scholars, so far, have written numerous books to prove his birth.
The text from : islampfr.com
