Based upon their belief of lutf (Grace), the Shi’ahs believe that it is incumbent upon Allah to send Prophets or their successors to this world to guide people towards the right path. The Sunnis say that it is not incumbent upon Allah, because they do not accept necessity of lutf.
The Shi’ahs and Sunnis in the first instance, and then the Sunnis among themselves, disagree about the theory of 'ismah (infallibility) of the Prophets.
What is our conception of ‘ismah? It is lutf (grace) of Allah which helps a person to refrain from sins, without effecting in any way his will and power. A ma’sum (sinless) person has the power to commit sins; but he does not even think about sins because his spiritual standard is so high that such inferior things do not enter his mind.
The Sunnis do not speak with one voice upon this subject.
They first differ about the point when ‘ismah begins. Some say it is after the declaration of Prophethood; others say that it is since childhood.
Second Difference: The scope of ‘ismah before declaration of Prophethood: Some say that it covers all sins; majority say that they are protected from kufr (infidelity) only.
Third Difference: The scope of ‘ismah after declaration of Prophethood; it is agreed that the Prophets could not tell a lie after Prophethood. But what about other sins? Some say that they could commit other sins either intentionally or unintentionally; but the majority say that they could commit it unintentionally, but not intentionally.
Fourth Difference: About minor sins: They say it was possible for the prophets to commit minor sins, even intentionally. But that they were protected from such minor sins which might have degraded them in the esteem of people.
The Shi'ah Ithna-‘asharis’ stand about ‘ismah is that all the Prophets were sinless and infallible; they could not commit any sin, whether major or minor, whether intentionally or unintentionally; and that they were ma’sum from the beginning of their life till their last breath.
Ash-Shaykh as-Saduq says about Prophets that:
"Their word is the word of God, their order is the order of God, their forbidding is the forbidding by God . . . And that the Chiefs of the Prophets are five - and they are (called) “ulul `azm” - and they are Noah, Abraham, Moses, Jesus and Muhammad (be blessings of Allah upon them all) and that Muhammad is their Chief and best of all."
The Shi'ahs say that Imam must be appointed by God; that appointment - may be known through the declaration of the Prophet or the preceding Imam.
The Sunnis say that Imam (or Caliph, as they prefer to say) can be either elected, or nominated by the preceding caliph, or selected by a committee, or may acquire power through a military coup. If he is elected, it is enough that one man should do bay’ah (allegiance) to him.
The Shi’ahs say: That Imam must be ma’sum (sinless).
The Sunnis say (including the Mu’tazilites) that ‘ismah is not a condition for caliphate. Even if he is a tyrant and has sunk himself in sins, Hanbalites, Shafi’ites and Malikites forbid people to rise against that caliph. They say that they should preserve.
The Shi’ahs say that Imam must possess above all such qualities as knowledge, bravery, justice, wisdom, piety, love of God etc. The Sunnis say it is not necessary. A person inferior in these qualities may be elected in preference to a person having all these qualities of superior degree.
The Shi’ahs say that ‘Ali was appointed by Allah to be the successor of the Prophet, and that the Prophet declared it on several occasions. The Sunnis say that the Prophet did not appoint anybody to be his successor.