The subject of the priesthood of Christ is now pursued as having either a contrast or an analogy to that of Aaron. Aaron, as priest, could have compassion on the ignorant, and on them that are out of the way; for that he himself also is encompassed with infirmity. (See also Mal. 2:4-7.)
When in this world, "in the days of his flesh," the Lord Jesus felt need, testings, temptations, weariness, being alone, being misunderstood, having His life threatened.
He learned obedience "by the things which he suffered; and being made perfect, he became the author of eternal salvation unto all them that obey him" (Heb. 5:8,9).
In Heb. 5:6 the 110th Psalm is applied to Christ. He is "a priest forever after the order of Melchisedec" by divine decree.