Hebrews 7:4-10

Just think how great [Melchizedek] was: Even the patriarch Abraham gave him a tenth of the plunder!  Now the law requires the descendants of Levi who become priests to collect a tenth from the people--that is, their brothers--even though their brothers are descended from Abraham.  This man, however, did not trace his decent from Levi, yet he collected a tenth from Abraham and blessed him who had the promises.  And without doubt the lesser person is blessed by the greater.  In the one case, the tenth is collected by men who die; but in the other case, by him who is declared to be living.  One might even say that Levi, who collects the tenth, paid the tenth through Abraham, because when Melchizedek met Abraham, Levi was still in the body of his ancestor (Hebrews 7:4-10 NIV).
  The authority of the priesthood of Melchizedek is secured even though it preceded the Levitical priesthood within the Mosaic covenant.  Abraham did not just give anyone a tenth of his plunder after all.  And Abraham, the grandfather of Levi, was blessed by Melchizedek, which illustrates the authority of Melchizedek.  Melchizedek's priesthood foreshadowed the Messiah in a more general way, yet Hebrews is explaining the nature of the Melchizedek priesthood even superseded the Mosaic priesthood.  How?  The priesthood at it's core requires reliance on God!  Just because a Levite was in the lineage of Aaron did not mean his heart was committed to God.  God called to the priesthood, as it states earlier in Hebrews 5:4, yet a person could accept or reject that calling.  The same holds true today with a callings from God.  And any calling from God requires a reliance on God for blessings to result.
  Again, looking back at Hebrews 5:4-10 in conjunction with this passage we learn the plan of God was for Jesus to be a forever high priest in the order of Melchizedek!  A person who receives Jesus, God the Son, will receive the forgiveness of sins from Him who is the High Priest.  Jesus gave Himself as the sacrifice.
Hunter Irvine