Twelver Shia Islam inherited the Mutazili case for divine justice and made it one of five usul al-din, the roots of religion that every believer is expected to affirm on the basis of reason rather than mere transmission, alongside tawhid, prophethood (nubuwwah), the imamate (imamah) and the resurrection (qiyamah). Where Sunni Ash'ari theology holds that an act is just purely because God commands it, so that the category of injustice could not even apply to Him, Twelver theology holds that justice is a real, rationally knowable standard God's own nature necessarily satisfies, and that describing God as just is meaningful precisely because injustice remains a coherent, if impossible, alternative.
The doctrine carries practical theological weight. It grounds the Twelver account of free will, since a God bound by justice cannot be said to compel human beings to sin and then punish them for it, a position Twelver theologians summarized in a formula attributed to the sixth Imam, Ja'far al-Sadiq: la jabr wa la tafwid, bal amr bayn amrayn, neither compulsion nor total delegation, but a matter between the two. It also underwrites the doctrine of the imamate, since a just God would not leave the community without a rightly guided leader, and it shapes Twelver eschatology, since a just God must eventually right the world's accumulated wrongs, an expectation bound up with the return of the Twelfth Imam from occultation.