Adopted from the book : "The Faith of Shi'a Islam" by : Allamah Muhammad Ridha al-Muzaffar"
We believe that, like the prophet, an Imam must be infallible, that is to say incapable of making errors or doing wrong, either inwardly or outwardly, from his birth to his death,either intentionally or unintentionally, because the Imams are the preservers of Islam and it is under their protection. Their position in regard to Islam is the same as the prophet and the reasoning which necessities their infallibility is the same difference between them in these matters.
An Arabic verse says:
"For Allah it is not impossible :
to unite all the world in one person"