Within Christianity, the early heresies all won popularity more by their ascetic [pietistic] and mystical practices than by any teachings they possessed. Almost all of them were either infiltrations of Eastern thought or the result of Greek mystic thought. Gnosticism, Mithracism, Neoplatonism, and later also Manichaeism all had Eastern origins. They sought the world’s regeneration in outdoing [historic orthodox] Christianity in ascetic and mystical efforts.

Likewise, Islam as a heresy of Judaism has produced some of the greatest spiritual poetry, while its Arab philosophers have profoundly influenced Western thought. Its spiritual masters have been among the greatest, the most severe, and the most faithful in the history of asceticism. Thus it is not just in doctrine that such Eastern faiths have challenged the West, but in their practices of spirituality.

Evangelical Dictionary of Theology, 1139