Hyssop represents common faith. Hyssop was a weed that grew practically anywhere. The blood is applied by common faith in God's word.
EXODUS 12:22 And ye shall take a bunch of hyssop, and dip it in the
blood that is in the bason, and strike the lintel and the two side
posts with the blood that is in the bason; and none of you shall go
out at the door of his house until the morning.
In many of the Old Testament sacrifices (the two turtle doves for the cleansing of the leper, and especially the sacrifice of the red heifer), hyssop was burned in the fire, with the sacrifice, along with scarlet and cedar.
Numbers 19:6 And the priest shall take cedar wood, and hyssop, and
scarlet, and cast it into the midst of the burning of the heifer.
Hyssop represents the first step of salvation, believing on the Lord through common faith (justification).
Scarlet represents the blood sacrifice which atones for our sins, and being set aside for service (sanctification).
Cedar represents a Holy God coming to dwell in our hearts (baptism of the Holy Ghost).
The reference to hyssop in John 19:29 ties together the Old Testament prefigure of the red heifer (who died for the sins of the people) with the death of Christ (who died "once for all"). There on Golgotha was Hyssop, the Blood Sacrifice (Scarlet), and Emmanuel (God with Us, Cedar).