This discovery of a “divine” essence or substance, dwelling, as Ruysbroeck says, at the apex of man’s soul is that fundamental experience—found in some form or degree in all genuine mystical religion—which provides the basis of the New Testament doctrine of the indwelling spirit. It is, variously interpreted, the “spark of the soul” of Eckhart, the “ground” of Tauler, the Inward Light of the Quakers, the “Divine Principle” of some modern transcendentalists; the fount and source of all true life.

At this point logical exposition fails mystic and theologian alike. A tangle of metaphors takes its place. We are face to face with the “wonder of wonders”—that most real, yet most mysterious, of all the experiences of religion, the union of human and divine, in a nameless something which is “great enough to be God, small enough to be me.” In the struggle to describe this experience, the “spark of the soul,” the point of juncture, is at one moment presented to us as the divine to which the self attains: at another, as that transcendental aspect of the self which is in contact with God.

On either hypothesis, it is here that the mystic encounters Absolute Being. Here is his guarantee of God’s immediate presence in the human heart; and, if in the human heart, then in that universe of which man’s soul resumes in miniature the essential characteristics.