7 Hyssop (אֵזוֹב, ʾēzôb) was also used for sprinkling blood (cf. Lev 14:4–8; Nu 19:6–8, 17–21; Heb 9:19; see Zohary, Plants of the Bible, 96–97).
7. Thou shalt purge me with hyssop. He still follows out the same strain of supplication; and the repetition of his requests for pardon proves how earnestly he desired it. He speaks of hyssop, in allusion to the ceremonies of the law; and though he was far from putting his trust in the mere outward symbol of purification, he knew that, like every other legal rite, it was instituted for an important end. The sacrifices were seals of the grace of God. In them, therefore, he was anxious to find assurance of his reconciliation; and it is highly proper that, when our faith is disposed at any time to waver, we should confirm it by improving such means of divine support. All which David here prays for is, that God would effectually accomplish, in his experience, what he had signified to his Church and people by these outward rites; and in this he has set us a good example for our imitation. It is no doubt to the blood of Christ alone that we must look for the atonement of our sins; but we are creatures of sense, who must see with our eyes, and handle with our hands; and it is only by improving the outward symbols of propitiation that we can arrive at a full and assured persuasion of it. What we have said of the hyssop applies also to the washings referred to in this verse, and which were commonly practised under the Law. They figuratively represented our being purged from all iniquity, in order to our reception into the divine favour. I need not say that it is the peculiar work of the Holy Spirit to sprinkle our consciences inwardly with the blood of Christ, and, by removing the sense of guilt, to secure our access into the presence of God.
51:7 You directed that hyssop and running water should be used in the ceremony for cleansing a leper (Lev. 14:1–8). Well, Lord, I take the place of a moral leper. Purge me with hyssop, and I shall be clean; wash me, and I shall be whiter than snow.
51:7 hyssop. Old Testament priests used hyssop, a leafy plant, to sprinkle blood or water on a person being ceremonially cleansed from defilements such as leprosy or touching a dead body (cf. Lv 14:6ff.; Nu 19:16–19). Here hyssop is a figure for David’s longing to be spiritually cleansed from his moral defilement. In forgiveness, God washes away sin (cf. Ps 103:12; Is 1:16; Mic 7:19).
51:7 hyssop. A plant with hairy leaves and branches; bunches of the branches are good for sprinkling. For its use in a cleansing ceremony, cf. Lev. 14:6; Num. 19:6. As with Ps. 51:2 (see note on vv. 1–2), the psalm highlights the inner condition to which the ceremonies point.
51:7 Hyssop alludes to cleansing ceremonies (Lev. 14:4; Num. 19:18) that point forward to the final cleansing from sin through the work of Christ (Heb. 9:19–28).
51:7 hyssop Israelites used hyssop branches to apply the blood of the Passover lamb to their doorposts (see Exod 12:22 and note). They were also used in other purifying rituals (Lev 14:49–53; Num 19:18–19).
I shall be whiter than snow Signifies the complete purity the psalmist wishes for (see Isa 1:18 and note).
51:7 with hyssop. The allusion is to Lev. 14:6, 7, where the cleansing of a leper is described.
wash me. This may be an allusion to Num. 19:19, where instructions are given for ritual washing after contact with a dead person. On the need for washing the heart, see “Legalism” at Matt. 23:4.
51:7 “Hyssop,” a small leafy shrub, was an integral element both in the purification of the leper (cf. Lev. 14:4, note) and also in removing the defilement resulting from contact with a dead body (cf. Num. 19:6). This verse is reflected in Is. 1:18.
 MacDonald, W. (1995). Believer’s Bible Commentary: Old and New Testaments. (A. Farstad, Ed.) (p. 630). Nashville: Thomas Nelson.
 MacArthur, J. F., Jr. (2006). The MacArthur study Bible: New American Standard Bible. (Ps 51:7). Nashville, TN: Thomas Nelson Publishers.
 Sproul, R. C. (Ed.). (2005). The Reformation Study Bible: English Standard Version (p. 781). Orlando, FL; Lake Mary, FL: Ligonier Ministries.
 Criswell, W. A., Patterson, P., Clendenen, E. R., Akin, D. L., Chamberlin, M., Patterson, D. K., & Pogue, J. (Eds.). (1991). Believer’s Study Bible (electronic ed., Ps 51:7). Nashville: Thomas Nelson.