From A History of Iran: Empire of the Mind, by Michael Axworthy (Basic Books, 2016), Kindle Loc. 2431-40, 2477-88:
The great schisms of the Christian church, between East and West, and later between Catholic and Protestant, came centuries after the time of Christ. But the great schism in Islam that still divides Muslims today, between Sunni and Shi‘a, originated in the earliest days of the faith—even before Karbala, in the time of the Prophet Mohammad himself. Comparisons with the Christian schisms do not really work. A more apposite analogy, as noted by the historian Richard N. Frye and others, can be drawn between, on the one hand, the emphasis on law and tradition in Sunni Islam and Judaism and, on the other hand, the emphasis on humility, sacrifice, and the religious hierarchy in Christianity and Shi‘ism. The public grief of Ashura is similar in spirit to that which one can still see on Good Friday in some Catholic countries. The purpose in making comparisons between Shi‘ism and various aspects of Christianity is not to suggest that they are somehow the same (they are not), nor to encourage some kind of happy joining-hands ecumenism (naïve), but rather to try to illuminate something that initially looks unfamiliar, and to suggest by analogy that it may not be so strange or unfamiliar after all. Or at least, that it is no more strange than Christian Catholicism.
Despite the schism, in the early centuries there was a fairly free interchange of ideas, a considerable pluralism of belief, and considerable diversity of opinion among the Alids or Shi‘a themselves. Overall, Shi‘a theology and law tended to be looser than in Sunni Islam, more open to the application of reason in theology, more inclined to a free will position than a determinist one, and more open to some of the more heterodox ideas circulating in the Islamic world. This was partly the result of a broader hadith tradition, which included the sayings and doings of the Shi‘a Emams. Shi‘a theology also differed because it addressed problems that were specific to the Shi‘a, such as conduct under persecution.
The sixth Shi‘a Emam, Ja‘far al-Sadiq, developed a strategy for the evasion of persecution that was to prove controversial. The doctrine of taqiyeh, or dissimulation, permitted Shi‘a Muslims to deny their faith if necessary to avoid persecution—a special dispensation that has striking similarities with the doctrine of “mental reservation” granted for similar reasons by the Catholic church in the period of the Counter-Reformation, and associated with the Jesuits (though it originated before their time). Just as the Jesuits acquired a reputation for deviousness and terminological trickery among Protestants (whence in English we have the adjective “Jesuitical”), so the doctrine of taqiyeh earned the Shi‘a a similar reputation among some Sunni Muslims.