And the priest shall command that one of the birds be killed in an earthen vessel over {c} running water:

(c) Running water, or of the fountains.

Wesley's Notes

14:5 Killed - By some other man. The priest did not kill it himself, because it was not properly a sacrifice, as being killed without the camp, and not in that place to which all sacrifices were confined. In an earthen - vessel - That is, over running water put in an earthen - vessel - Thus the blood of the bird and the water were mixed together, partly for the conveniency of sprinkling, and partly to signify Christ, who came by water and blood, 1John 5:6. The running water, that is, spring or river water by its liveliness and motion did fitly signify the restoring of liveliness to the leper, who was in a manner dead before.

Scofield Reference Notes

[2] vessel

The earthen vessel typifies the humanity of Christ, as the running water typifies the Holy Spirit as the "Spirit of life" Rom 8:2 "put to death in the flesh, but quickened by the Spirit." 1Pet 3:18

Jamieson-Fausset-Brown Bible Commentary

5-9. the priest shall command that one of the birds be killed . over running water-As the blood of a single bird would not have been sufficient to immerse the body of another bird, it was mingled with spring water to increase the quantity necessary for the appointed sprinklings, which were to be repeated seven times, denoting a complete purification. (See 2Ki 5:10; Ps 51:2; Mt 8:4; Lu 5:14). The living bird being then set free, in token of the leper's release from quarantine, the priest pronounced him clean; and this official declaration was made with all solemnity, in order that the mind of the leper might be duly impressed with a sense of the divine goodness, and that others might be satisfied they might safely hold intercourse with him. Several other purifications had to be gone through during a series of seven days, and the whole process had to be repeated on the seventh, ere he was allowed to re-enter the camp. The circumstance of a priest being employed seems to imply that instruction suitable to the newly recovered leper would be given, and that the symbolical ceremonies used in the process of cleansing leprosy would be explained. How far they were then understood we cannot tell. But we can trace some instructive analogies between the leprosy and the disease of sin, and between the rites observed in the process of cleansing leprosy and the provisions of the Gospel. The chief of these analogies is that as it was only when a leper exhibited a certain change of state that orders were given by the priest for a sacrifice, so a sinner must be in the exercise of faith and penitence ere the benefits of the gospel remedy can be enjoyed by him. The slain bird and the bird let loose are supposed to typify, the one the death, and the other the resurrection of Christ; while the sprinklings on him that had been leprous typified the requirements which led a believer to cleanse himself from all filthiness of the flesh and spirit, and to perfect his holiness in the fear of the Lord.

Matthew Henry's Concise Commentary

14:1-9 The priests could not cleanse the lepers; but when the Lord removed the plague, various rules were to be observed in admitting them again to the ordinances of God, and the society of his people. They represent many duties and exercises of truly repenting sinners, and the duties of ministers respecting them. If we apply this to the spiritual leprosy of sin, it intimates that when we withdraw from those who walk disorderly, we must not count them as enemies, but admonish them as brethren. And also that when God by his grace has brought to repentance, they ought with tenderness and joy, and sincere affection, to be received again. Care should always be taken that sinners may not be encouraged, nor penitents discouraged. If it were found that the leprosy was healed, the priest must declare it with the particular solemnities here described. The two birds, one killed, and the other dipped in the blood of the bird that was killed, and then let loose, may signify Christ shedding his blood for sinners, and rising and ascending into heaven. The priest having pronounced the leper clean from the disease, he must make himself clean from all remains of it. Thus those who have comfort of the remission of their sins, must with care and caution cleanse themselves from sins; for every one that has this hope in him, will be concerned to purify himself.