Psalms 51:7 - Coke's Commentary on the Holy Bible

Purge me with hyssop— תחטאני techatteeni: properly, expiate my sin, with hyssop. The Psalmist alludes to the purification from the leprosy; Leviticus 14:52 or from the touch of a dead body; Numbers 19:19 both which were to be done by the sprinkling of water and other things with hyssop. The Psalmist well knew that his sins were too great to be expiated by any legal purifications, and therefore prays that God would himself expiate and restore him through the great Sacrifice; i.e. make him as free from those criminal propensities to sin, and from all the bad effects of his aggravated crimes, as if he had been purified from a leprosy by the water of cleansing, sprinkled on him by a branch of hyssop, and that he might be, if possible, clearer from all the defilement and guilt of sin than the new fallen snow, through the Blood of the great Atonement. I think both these senses are included in the expiation which the Psalmist prays for; as the person whose leprosy was expiated was wholly cured of his disease, and freed from all the incapacities attending it.