From the Old Testament, we know that the priests were not exceptional models of morality and integrity. Aaron built the Golden Calf (Exodus 32-34), his sons Nadab and Abihu disobeyed God by offering unconsecrated incense. That was at the beginning of the priestly ministry. Later, in 1 Samuel we read that Eli’s sons, were also corrupt and were replaced by Samuel. The sins of the priests were one reason for the exile (Micah 3:11; Jer 1:18; Ezekiel 22:26). Psalm 110:4 promised a future messiah “The LORD has sworn and will not change his mind: "You are a priest forever, in the order of Melchizedek." The Ten Commandments were not just about God’s Law but a sign of his covenant with Israel. They were to be his people and he would be their God. But Israel broke this covenant over and over until God allowed them to be conquered and taken into exile. Yet, despite their unfaithfulness, during their exile, they get a wonderful promise. Jer 31:33 says "This is the covenant I will make with the house of Israel after that time, declares the LORD. I will put my law in their minds and write it on their hearts. I will be their God, and they will be my people.”
God does not change and, in his grace, God shows a future where his people will not break the new covenant because God will write the law on their hearts. The old covenant had been engraved in stone; the new covenant will include a revolutionary change in will, heart, and conscience. It will be an internal covenant. The law now becomes a principle of life (cf. Ro 8:1-4), a part of the nature of God's people. That is what we learn from Romans 8:1-4, “Therefore, there is now no condemnation for those who are in Christ Jesus, because through Christ Jesus the law of the Spirit of life set me free from the law of sin and death. For what the law was powerless to do in that it was weakened by the sinful nature, God did by sending his own Son in the likeness of sinful man to be a sin offering. And so he condemned sin in sinful man, in order that the righteous requirements of the law might be fully met in us, who do not live according to the sinful nature but according to the Spirit.” The core of the new covenant is God's gift of a new heart when we are born again. The motivation for obeying God's law is inner knowledge of his will, coupled with an enablement to perform it - all founded on the assurance that sins are forgiven.
The priests of Israel were but dim shadows of the one Great Priest of God, the Lord Jesus Christ. The writer of the book of Hebrews understood this perfectly. That is why he said confidently “They (meaning the priests) serve at a sanctuary that is a copy and shadow of what is in heaven. This is why Moses was warned when he was about to build the tabernacle, ‘See to it that you make everything according to the pattern shown you on the mountain.’ But the ministry Jesus has received is as superior to theirs as the covenant of which he is mediator is superior to the old one, and it is founded on better promises.” (Hebrews 8:5-6)
Therefore, throughout the book of Hebrews, we see the writer emphasizing the perfection found in the priestly character of Christ:
• Heb 5:5 - Christ as Priest is appointed and chosen by God
• Heb 7:20-22 - He is consecrated with an oath
• Heb 7:26 - He is sinless
• Heb 7:23-24 - His priesthood is unchangeable
• Heb 9:25-28 - His offering is perfect and final
• Heb 7:24-25 - He intercedes continually for his people.
So we can see that the priestly role of Christ is the ultimate fulfillment of the priestly ministry that began with Aaron and his sons. Looking at what we have learnt, in a sense, Christ did not supersede the priesthood although he did away with the old system of offerings. Just as Aaron is a type of Christ, we are also a type of Aaron’s sons who are to carry on Christ’s priestly work. Christ instituted the church, that is his body, to be the priests today. That is why 1 Pe 2:9 says “But you are a chosen people, a royal priesthood, a holy nation, a people belonging to God, that you may declare the praises of him who called you out of darkness into his wonderful light.” As Christians, we have a priestly role to play. We worship God as one body to proclaim the true God to a world that does not know him. We offer our prayers and our lives as sacrifices to show pre-believers what it means to live in a covenant relationship with God. We also offer God’s Word to people to help them find healing and blessing from God. That is what we do as a royal priesthood and holy nation – mediators between God and man.