HEBREWS:The Final and Only High Priest; "And having been perfected, Christ Jesus became the author of eternal salvation to all who obey Him, called by God as High Priest!"

Seeing then that we have a great High Priest who has passed through the heavens, Jesus the Son of God, let us hold fast our confession.  For we do not have a High Priest who cannot sympathize with our weaknesses, but was in all points tempted as we are, yet without sin.  Let us therefore come boldly to the throne of grace, that we may obtain mercy and find grace to help in time of need. (Hebrews 4:14-16)

For every high priest taken from among men is appointed for men in things pertaining to God, that he may offer both gifts and sacrifices for sins. He can have compassion on those who are ignorant and going astray, since he himself is also subject to weakness. Because of this he is required as for the people, so also for himself, to offer sacrifices for sins. And no man takes this honor to himself, but he who is called by God, just as Aaron was. (Hebrews 5:1-4)  

Priests are mere men, just like the rest of us.  They are not perfect!  They need forgiveness of their sins just like everyone else. ...if the anointed priest sins, bringing guilt on the people, then let him offer to the Lord for his sin which he has sinned a young bull without blemish as a sin offering. (Leviticus 4:3) Christ Jesus is the only person who ever lived that was able to live up to the calling of a High Priest, For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him. (II Corinthians 5:21)

So also Christ did not glorify Himself to become High Priest, but it was He who said to Him:

“You are My Son, today I have begotten You.” [Psalm 2:7]

As He also says in another place:

“You are a priest forever according to the order of Melchizedek”; [Psalm 110:4]

who, in the days of His flesh, when He had offered up prayers and supplications, with vehement cries and tears to Him who was able to save Him from death, and was heard because of His godly fear, though He was a Son, yet He learned obedience by the things which He suffered. And having been perfected, He became the author of eternal salvation to all who obey Him, called by God as High Priest “according to the order of Melchizedek,” of whom we have much to say, and hard to explain, since you have become dull of hearing. (Hebrews 5:5-11)

In order to become our High Priest, Christ first had to become our perfect sacrifice. Jesus prayed to God the Father before He was arrested and later crucified, a prayer which summarizes the scriptures above. And He was withdrawn from them about a stone’s throw, and He knelt down and prayed,  saying, “Father, if it is Your will, take this cup away from Me; nevertheless not My will, but Yours, be done.”  Then an angel appeared to Him from heaven, strengthening Him. And being in agony, He prayed more earnestly. Then His sweat became like great drops of blood falling down to the ground. (Luke 22:41-44, Mark 14:35) 

God's plan of salvation was all inclusive in His Son Christ Jesus, who obeyed the Will of God, even unto death, so that we all may be saved!  Even the high priest of the Jews prophesied this before Jesus was crucified. And one of them, Caiaphas, being high priest that year, said to them, “You know nothing at all, nor do you consider that it is expedient for us that one man should die for the people, and not that the whole nation should perish.” Now this he did not say on his own authority; but being high priest that year he prophesied that Jesus would die for the nation, and not for that nation only, but also that He would gather together in one the children of God who were scattered abroad. (John 11:49-52)

But who is Melchizedek?...