For the spiritual power of a sacrament is like light in this way: it is both received pure by those to be enlightened, and if it passes through the impure it is not defiled.

Tractates on the Gospel of John; tractate V on John 1:33, §15; translation by R. Willems | As quoted in Familiar Quotations, 9th edition (1892) edited by John Bartlett, p. 169 | Comparable to | The sun, too, shines into cesspools and is not polluted. | Diogenes Laërtius, Lib. vi. section 63 | A very weighty argument is this — namely, that neither does the light which descends from thence, chiefly upon the world, mix itself with anything, nor admit of dirtiness or pollution, but remains entirely, and in all things that are, free from defilement, admixture, and suffering. | Julian, in Upon the Sovereign Sun, (c. December 362), as translated by C. W. King in Julian the Emperor (1888) - Full text online | The sun, which passeth through pollutions and itself remains as pure as before. | Francis Bacon, Advancement of Learning, Book II (1605)

The spiritual virtue of a sacrament is like light : although it passes among the impure, it is not polluted.