Sufism is commonly considered as the esotericism of Islam. It appears to be the extension, in the sense of piety, asceticism and mysticism, of the common formal religion. But to reduce it to this aspect alone would be to miss its true nature. In fact, Sufism is the quintessence of religious and spiritual symbolism and, by the same token, exceeds it. This quintessence is metaphysical and universal. It is not esotericism, it contains it. Of course, Sufism is based on the fundamental symbols of Islam. But by returning to their archetypal, and therefore universal, intentions, it reaches what can be called the sophia perennis. Thus, Sufism presents itself in a veiled manner before its quintessence appears, resulting from the very nature of things, which coincides with all metaphysics and all contemplative spirituality, whatever their traditional origin.
Christentum – Islam: Ausblicke auf eine esoterisch Ökumene (e-Book)
In this work, Frithjof Schuon compares Christianity and Islam and also looks at confessions within these world religions: Protestantism, Orthodoxy, and Shiism.