Melchizedek was the king of Salem, later Jerusalem, around 2000 B.C., and a high priest after the holy order of God. His people had grown strong in iniquity and abomination and had all gone astray, but Melchizedek exercised mighty faith, received the office of the high priesthood, and preached repentance to them. They repented, and he established peace in the land, for which he was called the prince of peace; he reigned under his father (Alma 13:17-18). Abraham paid him tithes, one-tenth of all he possessed (Alma 13:15). The ordinances of this order were given so that the people might look forward to the Son of God for a remission of their sins (Alma 13:16).
The higher priesthood, originally called “the Holy Priesthood, after the Order of the Son of God,” was renamed after Melchizedek to avoid too frequent repetition of the name of the Supreme Being (D&C 107:2-4). His people merited translation and joined the city of Enoch (JST Genesis 14:34).
Alma the Younger said that of the high priests there were many before Melchizedek and many afterward, but none were greater, and that of him they have more particularly made mention (Alma 13:19).