“But others seem pale of body and shadowed of soul. For these are weak in faith and do not firmly believe in this sacrament, but find it as hard to understand wisdom as a child whose deeds are done in foolishness. They hear outwardly with the hearing of the ear and perceive with sluggish hearts what is said to them about the sacrament, and would gladly embrace it in perfect faith; but because of the doubt that is in them they cannot understand the great holiness it has. Therefore too their inner being is plunged in darkness, for they cannot lift their minds to that perfection; conceived in sin, they are too borne down by the weight of bodily weakness to be perfect in belief, though their spirit may consent to it, and they would gladly have understanding of faith if their apathy of heart would let them.”
Source: St. Hildegard of Bingen, Hildegard of Bingen: Scivias (Classics of Western Spirituality (Paperback)), Paulist Press, 1990.