Origin of the Shia, Hossein Nasr, S.H.M Jafri
Shi’ism was not brought into existence only by the question of the political succession to Muhammad as so many Western works claim (although this question was of course of great importance).
The problem of political succession may be said to be the element that crystallized the Shi’ites into a distinct group, and political suppression in later periods, especially the martyrdom of Imam Husayn -upon whom be peace- only accentuated this tendency of the Shi’ites to see themselves as a separate community within the Islamic world.
The principal cause of the coming into being of Shi’ism, however, lies in the fact that this possibility existed within the Islamic revelation itself and so had to be realized.
Inasmuch as there were exoteric [Zaheri] and esoteric [Bateni] interpretations from the very beginning, from which developed the schools (madh’hab) of the Sharia and Sufism in the Sunni world, there also had to be an interpretation of Islam, which would combine these elements in a single whole.
This possibility was realized in Shi’ism, for which the Imam is the person in whom these two aspects of traditional authority are united and in whom the religious life is marked by a sense of tragedy and martyrdom… Hence the question which arose was not so much who should be the successor of Muhammad as what the function and qualifications of such a person would be.
Hossein Nasr, Shi’ite Islam,
Those who thus emphasize the political nature of Shi’ism are perhaps too eager to project the modern Western notion of the separation of church and state back into seventh century Arabian society, where such a notion would be not only foreign, but completely unintelligible. Such an approach also implies the spontaneous appearance of Shi’ism rather than its gradual emergence and development within Islamic society.
Jafri, S.H Mohammad. “The Origin and Early Development of Shi’a Islam,”, Oxford University Press, 2002,