For every high priest chosen from among men is appointed to act on behalf of men in relation to God, to offer gifts and sacrifices for sins. He can deal gently with the ignorant and wayward, since he himself is beset with weakness. Because of this he is bound to offer sacrifice for his own sins as well as for those of the people. And one does not take the honor upon himself, but he is called by God, just as Aaron was.
So also Christ did not exalt himself to be made a high priest, but was appointed by him who said to him,
“Thou art my Son,
today I have begotten thee”;
as he says also in another place,
“Thou art a priest for ever,
after the order of Melchiz′edek.”
In the days of his flesh, Jesus offered up prayers and supplications, with loud cries and tears, to him who was able to save him from death, and he was heard for his godly fear. Although he was a Son, he learned obedience through what he suffered; and being made perfect he became the source of eternal salvation to all who obey him, being designated by God a high priest after the order of Melchiz′edek.
–Hebrews 5:1-10