The Shiah counter that the imam is in no way equal to God. Like Catholic
saints, he is merely set apart from the rest of humanity. The imam may be
prayed to for intercession, and he may have the power to heal the sick, but
his authority is derived solely from his connection to the Prophet. And just
as there are a fixed number of prophets, ending with Muhammad, so are there
a fixed number of imams, ending with "the Hidden Imam," known as the Mahdi.

Nearly all Muslims acknowledge the existence of the Mahdi, a messianic
figure who will return at the End of Days to usher in a time of peace and
justice. Sunni and Shiah alike believe the Mahdi's coming will be an
apocalyptic event portended by earthquakes, wars, famine, and false
prophets. In Islam, the Mahdi's return will herald the return of Jesus; both
prophets will rule the next world together.

However, as the Shiah shaped the doctrine of the Mahdi into the central
tenet of their faith, Sunni scholars began to distance themselves from
further speculation on the topic in an attempt to separate themselves from
what fast became a politically disruptive ideology. That's because according
to the Shiah, the Mahdi's principal task upon returning to earth will be to
avenge the injustice inflicted by the Sunni authorities upon Husayn and his
followers at Karbala. [Slate]

All religious factionalism seems foolish and arbitrary from the outside - this fundamental antagonism between the two main branches of islam would seem to be an excellent argument for separation of church and state.