Nadir Shah’s religious policy was a curious one. On his accession he repudiated the Shi'a faith, which since 1500 had deeply penetrated Persian life. He later attempted a crude assimilation of Shi'ism into Sunni Islam by trying to persuade or compel the Sunni doctors to admit Shi'ism as a fifth system of orthodox law. This policy was an utter failure, since neither Sunni nor Shi'a divines would agree to such a politic burying of their differences.
Nadir perhaps was aiming at a pan-Islamic empire, hoping to unite Turkey and India into one gloriously restored caliphate; and he may have thought it necessary to reconcile the two hostile factions of Islam as an essential preparation for the achievement of this ideal. In personal life he appears to have been completely, almost naively, irreligious.
William H. McNeill. The Rise of the West: A History of the Human Community; with a Retrospective Essay. 1992. p. 903.