Hebrew Roots/Holy Priesthood
The Dictionary definition of a priest is "one especially consecrated to the service of a divinity and through whom worship, prayer, sacrifice, or other service is offered to the object of worship - and pardon, blessing, or deliverance is obtained by the worshipper." (Funk and Wagnall Vol. 21 p.273)
The first priest on earth was Adam and the concept of priesthood has not changed from its first institution with him. It is only the order of priesthood which has changed. There have been several changes in the form of priesthood which Yahweh God has ordained for man to approach Him and with each change in priesthood a change came in the sacrifices that were offered and the type of "tabernacle" in which they were offered. A priesthood is always established on a covenantal agreement between the worshippers and their God, and that is inaugurated by a sacrificial offering.
A priest is required to act as a mediator. He is one who represents the Divine being to His subjects and in return from them to their God. He acts as an ambassador, a chosen vehicle through whom Yahweh God has chosen to serve the people and represent Him, on His behalf. He also mediates between the people and their God.
The Melchizedek Priesthood 
Sonship in the Melchizedek Order