Two things set the high priest apart from the other priests: his ornate robes, and the beautifully fragrant smell of the oil with which he had been anointed. Once he inherited the office from his father, he could not lay it aside; he was dedicated to the service of God for the rest of his life.
The high priest played a key role in certain ceremonies, especially the Day of Atonement – when he was the only person allowed to enter God’s immediate presence, by passing behind the curtain of the Tabernacle into the Most Holy Place. Over his chest he bore twelve gemstones, representing the whole nation of Israel; thus, symbolically, all the people of God entered the Tabernacle with him.
Only those who are perfect can safely approach a holy God, so no-one with a congenital malformation or physical defect could fulfil a priestly role (Leviticus 21:16-23). Even so, complete perfection and holiness is beyond the reach of any human being. Aaron himself was far below the necessary standard (for he had been implicated in the making of the golden calf-idol). So his office, vitally important though it was, could only be symbolic. There is no-one worthy to reconcile the world to God or able to intercede effectively for the human race – except Christ (Revelation 5:1-5). “Such a high priest truly meets our need – one who is holy, blameless, pure, set apart from sinners, exalted above the heavens.” (Hebrews 7:26)
Jesus was not descended from Aaron; in fact, He was not even a member of the tribe of Levi. How then could He act as High Priest for us? By being appointed under a different (and much older) system: the ancient order of the priest-kings of Jerusalem, the order of Melchizedek (Psalm 110:4; Hebrews 7:1-3). Nevertheless, His ‘ordination’ has many similarities with Aaron’s. Just like the high priest, Christ’s ministry was inaugurated by baptism in water and by anointing with the Holy Spirit (Mark 1:10).
Now imagine the scene in the shadow of Mount Sinai… Aaron the first high priest, having completed the seven-day ordination ritual, initiates the worship of God’s people under the old covenant (Leviticus 9). He offers sacrifices: sin offerings, burnt offerings, fellowship offerings. Then he goes into the Tabernacle (where the assembled crowd cannot see him) to spend time with God. When he emerges back into the light, he raises his hands in blessing – and supernatural fire comes from God’s presence, enveloping the offerings on the altar and causing the people to prostrate themselves with awe. What they have seen acted out before them is what our great High Priest will do: He will make atonement for our sins (by offering His own body, on the cross), come back to bless His disciples (Luke 24:50) – and then, shortly afterwards, the fire of the Holy Spirit will come from heaven to initiate the worship of the new covenant (Acts 2:1-4).