12 “You shall sprinkle me,” he says, “with hyssop, and I shall be cleansed”. Hyssop we know to be a herb humble but healing: to the rock it is said to adhere with roots. Thence in a mystery the similitude of cleansing the heart has been taken. Do thou also take hold, with the root of your love, on your Rock: be humble in your humble God, in order that you may be exalted in your glorified God. You shall be sprinkled with hyssop, the humility of Christ shall cleanse you.
Despise not the herb, attend to the efficacy of the medicine. Something further I will say, which we are wont to hear from physicians, or to experience in sick persons. Hyssop, they say, is proper for purging the lungs. In the lung is wont to be noted pride: for there is inflation, there breathing. It was said of Saul the persecutor as of Saul the proud, that he was going to bind Christians, breathing slaughter: he was breathing out slaughter, breathing out blood, his lung not yet cleansed.
Hear also in this place one humbled, because with hyssop purged: “You shall wash me,” that is, shall cleanse me: “and above snow I shall be whitened.” “Although,” he says, “your sins shall have been like scarlet, like snow I will whiten.” Out of such men Christ does present to Himself a vesture without spot and wrinkle. Further, His vesture on the mount, which shone forth like whitened snow, signified the Church cleansed from every spot of sin.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)