Based upon their belief of lutf (Grace), the Shias believe that it is incumbent upon Allah to send Prophets or their successors in this world to put people on right path.
Sunnis say that it is not incumbent upon Allah, because they do not accept necessity of lutf.
Shias and Sunnis in first instance, and then Sunnis among themselves, disagree about the theory of 'ismah (sinlessness) of the prophets.
What is our conception of Ismah? It is lutf (Grace) of Allah which helps a person to refrain from sins, witllout effecting in any way his will alld power. A Ma'sum (sinless) person as power to comrnit 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. However Sunnis do not speak with one voice upon this subject:
The First difference: Is about the point when Ismah begins. Some say it is after the declaration of prophethood; and others say that it is since childhood.
Second Difference: Scope of Ismah before declaration of prophethood: Some say that it covers all sins; majority says that they are protected from kufr (infedility) only.
Third Difference: 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 says that they could commit it unintentionally, but not intentionally.
The 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 infallable; they could not commit any sin, whether capital or minor, and whether intentionally or unintentionally; and that they were ma'sum from the beginning of their life till their last breath.
Shaykh al-Saduq says about the prophets that:
"Their word is the word of God, their order is the order of God, their forbiding is the forbiding by God And that the Chiefs of the prophet are five and they are (called) Ulul’azm - and they are:
Be the Blessings of Allah upon them all!
And that The Holy Prophet Mohammad(pbuh&hf) is their Chief and the best of them all." I'tiqadat
The Shias say, "That Imam must be appointed by God; that appointment may be known through the declaration of The Prophet or the preceeding Imam."
Sunnis say that Imam (or Caliph, as they prefer to say) can be either elected, or nominated by the preceeding 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 Shias say: "That Imam must be Ma'sum (sinless)."
Sunnis say (including Muitazilites) that Ismah is not a condition for caliphate. Even if he is tyrant and sunk in sins, Hanbalites, Shafi'ites and Malikites forbid people to rise against that Caliph. They say that they should persevere.
The Shias say, "That Imam must possess above all such qualities as knowledge, bravery, justice, wisdom, piety, love of God etc."
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 Shias say, "That Ali Ibin Abe Taleb(as) was appointed by Allah(swt) to be the successor of The Prophet(pbuh&hf), and that The Prophet(pbuh&hf) had declared it on several occassions.
Sunnis say that The Prophet did not appoint anybody to be his successor.