Sufism then spread to Persia. But whenever the Sufis expressed their free thought, they were attacked by the established religions, and so Sufism found its outlet in poetry and music. Thus it happened that the great Sufi poets, such as Hafiz, Rumi, Shams-i Tabriz, Sadi, Omar Khayyam, Nizami, Farid, Jami, and others, gave the wisdom of Sufism to the world. Rumi's work is so great that if one has read and understood it one has learnt every philosophy there is. His poems are sung in the sacred assemblies of the Sufis as part of their worship. The lives of the Sufis were marvelous in their piety, in their humanity.