Purge me with hyssop, and I shall be clean: wash me, and I shall be whiter than snow (Psalm 51:7).
The metaphor, white as snow, is used several times in Scripture in connection with three great themes. First of all, it is associated at least three times with the dread disease of leprosy. The first such instance was when God made the hand of Moses leprous as snow, but then quickly restored it (Exodus 4:6). Secondly, Moses sister Miriam became leprous, white as snow when she and Aaron spoke against Moses (Numbers 12:10). Then Gehazi, Elishas servant, because of his covetousness, was similarly smitten and went out from (Elishas) presence a leper as white as snow (II Kings 5:27).
But the phrase not only speaks of sin in terms of leprosy (which is a type of sin), but also of salvation from sin. Isaiah 1:18 is a beautiful text. Come now, and let us reason together, saith the LORD: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool. David, in his famous prayer of repentance after his terrible sin in the matter of Uriah and Bathsheba, spoke of being purged with hyssop, a small shrub used to sprinkle blood and water over a leper that had been cleansed, symbolizing his purification and cleansing (Leviticus 14:47). Then he prayed, wash me and I shall be whiter than snow (Psalm 51:7).
Finally the phrase is used not only to refer to sin and salvation, but also to the Savior in His glorified state. His head and His hairs were white like wool, as white as snow (Revelation 1:14). . . . and He was transfigured before them. And His raiment became shining, exceeding white as snow (Mark 9:2,3). Snow, therefore, can remind us of the shame of sin, but also of the glory of Christ and the joy of salvation. HMM