A ray of sunlight shining on a smudgy window is unable to illumine that window completely and transform it into its own light. It could do this if the window were cleaned and polished. The less the film and stain are wiped away, the less the window will be illumined; and the cleaner the window is, the brighter will be its illumination.
The extent of illumination is not dependent on the ray of sunlight but on the window. If the window is totally clean and pure, the sunlight will so transform and illumine it that to all appearances the window will be identical with the ray of sunlight and shine just as the sun’s ray.
Although obviously the nature of the window is distinct from that of the sun’s ray (even if the two seem identical), we can assert that the window is the ray or light of the sun by participation. The soul on which the divine light of God’s being is ever shining, or better, in which it is ever dwelling by nature, is like this window, as we have affirmed.
A soul makes room for God by wiping away all the smudges and smears of creatures.
Saint John of the Cross
The Ascent of Mount Carmel, II.5.6–7
John of the Cross, St. 1991, The Collected Works of St. John of the Cross, Revised Edition, translated from the Spanish by Kavanaugh, K and Rodriguez, O with revisions and introductions by Kavanaugh, K, ICS Publications, Washington DC.