Hebrews 5: 1 - 6
1 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.
2 He can deal gently with the ignorant and wayward, since he himself is beset with weakness.
3 Because of this he is bound to offer sacrifice for his own sins as well as for those of the people.
4 And one does not take the honor upon himself, but he is called by God, just as Aaron was.
5 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";
6 as he says also in another place, "Thou art a priest for ever, after the order of Melchiz'edek."

EWTN