High priest are selected by men to represent them to God as well as offer sacrifices for their sin to God.  God chooses the high priest, like Aaron, God also chose Jesus as it was testified to in Psalms.  Jesus as son of God offered prayers and petition to God, and it was after he had perfected is obedience unto death that God raised him to become external salvation for all who had faith in Jesus.  God made him a high priest under Melchizedek's (king and high priest of Salem (Jerusalem), who took an offering from Abraham on the future promised land) order. The matured can distinguish the wisdom of God's good from evil.

Hebrews 6:

Let us move on to the matured truth of God in Jesus based on scriptures.  It is very difficult to bring back to repentance the faithful in Christ who were once filled with the Holy Spirit, seen the goodness of God's word, and powers of the coming age (God's kingdom in Jesus) after they fall way from faith in Christ because it is to crucify Christ again in disgrace.  Land that produces good harvest are a blessing to farmer from God, but the one that produces thorns are a curse to the farmer and it will be burnt.  Maintain your faith and good works in Christ to the end because God is just as well as remembers your love for people of God in Jesus.  God will faithfully reward those who are faithful to the end.  God swore by himself when he made a promise to Abraham because none is greater and God is unchanging. 
 

Hebrews 7:

Melchizedek (king and high priest of Salem (Jerusalem)) took tithe from Abraham on all the plunder he got from war against the kings.  Melchizedek has no parent or ancestry, but his name means "king of Righteousness" and King of Salem means "king of peace". Melchizedek was quite great for Abraham to give him tithe and by law the Levites had to collect tithe from the other 12 tribes (Joseph was split into Manasseh & Ephraim).  Melchizedek was not a descendants of Levi yet collected tithe from Abraham and blessed Abraham who had the promise of God, so he was greater than Abraham.  The Jewish law under Aaron was not perfect, so another covenant was need under Melchizedek through Jesus, which is a new change. Jesus is from Judah and not Levi, as commanded by the Jewish law, and David in Psalms said that Jesus is priest forever under Melchizedek, which is based on the power of an everlasting life and not Jewish ancestral laws. Judaism and the Jewish law is hopeless and useless in getting us closer to God.  Others become priest by ancestry, but Jesus became priest by an oath and God declared it in Psalms.  Unlike other priest who die and offer sacrifice daily, also for themselves to cleanse their sins, Christ lives forever and made his sinless sacrifice once and for all.
 

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