One or two notes taken from a sermon by one of our ministers some years ago. This is a summary not a verbatim quote.
Psalms 51v7 "Purge me with hyssop, and I shall be clean: wash me, and I shall be whiter than snow".
This is one of the seven penitential Psalms. Augustine asked for all seven to be put on the walls of the room where he lay in his final illness.
Hyssop was a plant used to sprinkle blood and water in the various ceremonies of the Old Testament. It was used for example to confirm that the leper -- formerly kept outside the camp -- was now cured and therefore clean. That is a picture of the saved sinner. The priests used hyssop in the sacrifices. And it was also used by Moses in making the covenant with the Most High on behalf of the people. In this Psalm, David is seeking that the Lord would expiate -- put away -- the guilt and pollution and power of sin. The word purge in this verse was translated in Luther's German Bible by a word not found normally in English -- "un-sin me". All of these points have relevance to the sacrament of baptism. The sprinkling is a symbol, a sign, something that signifies the inner cleansing that must take place in the sinner. The person baptised is under the vows and in adult life we should all be considering whether we have fulfilled the blessings available in the visible church, of which we become part by baptism.