Melchizedek was both…king of Salem (the early name for Jerusalem) and high priest of the Most High God.  In these verses Jesus is also being shown to have combined both kingly and priestly duties.  For it tells us in Hebrews that Jesus entered the Most Holy Place, once for all, by way of His own blood and for our sake, and made atonement for the sins of all people.  And in doing so, we can now have this eternal hope…secure and reliable, that will never fail.