Since the Aaronic or Levitical priesthood would come to an end after the crucifixion of Christ, and the sanctuary on earth would also come to an end, God planned to make the Lord Jesus Christ our great High Priest in the Sanctuary in Heaven.
However, since Messiah was to come from the tribe of Judah, God set up a special priestly *order* for Him -- the order (appointed succession, position, or rank) of Melchizedek.
Melchizedek was a mysterious King-Priest who met with Abraham, and to whom Abraham offered a tenth of the spoils which he had taken from the battle of the kings in Genesis. There has never ever been a King-Priest, and this Melchizedek had neither father nor mother, neither beginning of days nor end of life. So the logical conclusion is that Melchizedek (King of Righteousness, King of Peace) was none other than the pre-incarnate Christ Himself. Study the epistle to the Hebrews.