A Priest (Heb. nhk cohen; Gr. iereus < ierus) is a person consecrated to a deity, often with duties of intercession and sacrifices on the behalf of others. Both the Hebrew and Greek terms come from iniquity, with neither having any "root" word pointing to anything other than their function. The belief in God in all ancient cultures, depended upon a mediator. While Priests were most notable for their sacrificial duty, anyone involved in religious ceremonies was considered a priests. Despite their special duty, Priests often became corrupt and often did not follow through with the ceremonies they were to perform.
In the beginning, everyone seems to have offered sacrifices individually, though this seems to have very soon passed to the head of households. Though Cain and Abel offered sacrifices with no reference to family or clan, by the time of Adam's grandson Enos, the duty probably had fallen upon the patriarchs — the heads of the families or clans. As heads of their households Noah, Job, Abraham, Isaac and Jacob all offered sacrifices on their families' accounts. In the time of Abraham, the first priest to be mentioned in the Bible is a man named Melchizedek. "the priest of the Most High God."
While the term "priest" ubiquitously refers to the office, Scripture also assigns it an unofficial meaning. A priest is anyone who is specially consecrated to be set apart for God. The Office of Priest would be exemplary of someone set apart through the special ceremonies that no one else had to perform. This is why the whole nation of Israel was referred to as a "royal priesthood". Ultimately Jesus Christ would become the final High Priest, heading the priesthood of Christians.
Order of Melchizedek
In the beginning, everyone seems to have offered sacrifices individually, though this seems to have very soon passed to the head of households. Apparently sacrifices had been happening since Cain and Abel, but the Bible does not record God's ordination of this. As heads of their households Noah, Job, Abraham, Isaac and Jacob all offered sacrifices on their families' accounts.
By the time this happened, false religions have developed . The first Yahwistic priest mentioned in the Bible is Melchizedek. Melchizedek had a dual role as monarch over the city of Salem. Melchizedek is given the priestly epithet of "Priest of the Most High God" (Cohen El Elyon) perhaps to distinguish him from priests of false gods. The exact responsibilities of Melchizedek's position, how he received it or the existence of previous priests is unknown. As a priest Melchizedek did bring a meal of bread and wine for Abraham.
It is possible he was the first priest, based on an analysis given by the author of Hebrews and from a Psalm written by David. Melchizedek apparently had his own priesthood, called the Order of Melchizedek (which only him and Jesus were apart of).
Priests of Midian
There is also another Pre-Levitical Priest that was of God, Reuel, the father-in-law of Moses. It is possible he was from a whole line of priests from his nation of origin, Midian. Midian's founder was born to Keturah, the wife of Abraham; thus the Midianites descended from a Hebrews. In addition Reuel's name ("Friend of God") further supports and strengthen's him as a priest, perhaps by birth. Like Melchizedek he is given the epithet of "Priest of the Most High God" to distinguish from false priests.
Shortly after Israel's independence from Egypt, Reuel served as a judicial advisor to Moses. As a priest he made a sacrifice to God via fire. Reuel also devised a judicial system for the new nation of Israel to free Moses up from works as a judge. He also instructed Moses to teach all of the Israelites the commands of God's law, paving the way for Israel being an obedient nation.
Sons of Aaron
A short time after this God ordered and specified the construction of the Tabernacle, which would be the designated place for all sacrifices and religious ceremonies; additionally it would be the headquarters of the priesthood. Within the instructions for the Tabernacle were the instructions for the various altars where sacrifices would be placed.
Once some basic laws, including the Ten Commandments and establishment of the festivals; as well as the Tabernacle and altars were built, specially ordained workers were needed to operate and enforce the ceremonies. So on Mount Sinai, God asked Moses to appoint his brother Aaron and his sons, Nadab, Abihu, Eleazar and Ithamar as Priests. They were made Priests since they were part of the Tribe of Levi, which had prophetically been ordained to be the priesthood. Special clothes were made for Aaron and his sons, in order that they may be consecrated and could serve in God's presence as priests. This would be the first ordinance that would be passed on through the generations of the Tribe of Levi.
Many more laws and ordinances would be put in place that assigned special functions to Aaron and his sons, which forbade any other Israelite from performing certain roles. The priestly garments and the laws were made, prior to Aaron and his sons beginning their work as priests.
Ordination and Consecration
At one point in time God summoned Aaron and his sons before him to be consecrated, per the regulations that had been given sometime prior. The necessary sacrifices in addition to the priestly garments were brought for the ceremony. Moses gave the sacrifices and did the necessary ceremonies with water and bread to Aaron and his sons before all of Israel. Then Aaron and his sons were commanded to stay at the entrance of the Tabernacle for seven days and night, as part of their consecration ceremony.
This process would symbolically separate the Priests from the common people, marking them as holy and specially consecrated to God.
The First Sacrifices
On the eighth day since the sacrifices for consecration, Aaron and his sons along with Israel's elders were summoned outside the Tabernacle. Moses commanded the Priests to take sin, burnt and fellowship offerings as atonement for themselves and the people. Aaron and his sons went into the Tabernacle before the offering and made the appropriate sacrifices. As the High Priest (the leader of the Priests) Aaron put the animals on the altars, while his sons prepared them and their blood. At this time there were five priests, performing sacrifices on behalf of over six-hundred thousand Israelites. 
After this Aaron and his sons came out of the Tabernacle. Shortly thereafter, Aaron returned inside the Tabernacle with Moses to convene with God. Once the Speaker and the High Priest returned outside, God set the sacrifices on fire and burned them and the people rejoiced.
Within hours after God had burned the first sacrifices, two of the priests: Nadab and Abihu decided to experiment with the altar (perhaps while drunk). They went up to the altar and burned fire, without the authorization of the Lord. Consequently, flames from God leaped at the two priests and they were burned to death. This reduced the entire priesthood to three members.
As God's Prophet and Mediator with God, Moses spoke with Aaron and explained that God could not tolerate unholiness. Moses ordered for the remaining priests, Eleazar and Ithamar (still in their priestly uniforms) to be brought outside the Israelite camp for a private discussion. Moses gave them orders to continue to maintain a proper appearance and to wait by the front of the Tabernacle until they were dismissed. Meanwhile God spoke to Aaron and told him Priests were no longer allowed to consume alcohol when they go inside the Tabernacle; this was a new rule that would apply for the rest of the Priesthood.
Priest of the Most High God
Melchizedek arose "out of nowhere" to meet with Abraham (then still called Abram) after the patriarch had successfully rescued Lot and others from the marauding forces of a local confederation of kings. The man was not only a priest, but he was the king of a city-state called Salem. As pointed out in the letter to the Hebrews, his name and his office were significant. His name meant "king of righteousness" and his realm was the town of Salem, which means "Peace." As king, this man assured the right thing would be done in a peaceful manner.
In all likelihood, Abraham's son by Keturah, Midian, was a follower of the one true God, El Elyon (The Most High God) and brought the religion to the southern parts of the Arabian Peninsula where years later Moses would find a wife, the daughter of Reuel Jethro, the priest of Midian. Having held onto the religion of his parents, Moses had drifted far from his roots. For forty years Moses worked for this man, probably becoming reacquainted to the God of his fathers.
Priest of Yahweh
When Yahweh revealed Himself to Moses, the old shepherd was afraid to speak on His behalf to the pagan leadership of Egypt. As as a result, the office of priest was given to his brother Aaron who would relay God's message which Moses received in his office as a prophet.
The whole tribe of Levi was separated especially for service to God. Though this included women and children, the consecration seems to have been of men having reached young adulthood upwards. Of these men, the priesthood would be reserved for Aaron and his descendants. The others would become "servants" of God, designated to take care of the property of the tabernacle, and later, of the temple.
- ↑ 1.0 1.1 Gen 8:20; 22:2, 31:54, 35:14 ;Job 1:5, 42:8
- ↑ Exo 19:6, 1 Peter 2:9, Rev 1:6, Rev 5:10, Rev 20:6 (Link)
- ↑ Gen 4:3-4 (Link)
- ↑ Gen 14:18, Heb 7:1 (Link)
- ↑ Psa 110:4, Heb 5:6. Heb 5:10, Heb 6:20, Heb 7:11, Heb 7:17 (Link)
- ↑ Exo 2:16, 3:1, 18:1 (Link)
- ↑ Gen 25:1-4 (Link)
- ↑ Exo 18:3 (Link)
- ↑ Exo 18:14-23 (Link)
- ↑ Exo 28:1 (Link)
- ↑ Exo 39:1-30 (Link)
- ↑ Exo 28:2, 4, 29-30, 43 (Link)
- ↑ Lev 8, Exo 29:1-37 (Link)
- ↑ Lev 9:1-22 (Link)
- ↑ Lev 9:23-24 (Link)
- ↑ Lev 10:4-11 (Link)