Hebrews 5 contrasts human priests with Jesus Christ as High Priest. Verse 5 continues the point from earlier in the chapter that the high priesthood is only by appointment by God. The human high priest did not put himself forward for appointment. Likewise, the author points out that Jesus did not glorify or exalt Himself in order to be High Priest. That office was given to Him by the Most High.
In verses 5-6, the author quotes Psalm 2 and Psalm 110 to show that Someone greater—the Most High—commanded things regarding Jesus, including His role as High Priest. That shows that Jesus did not glorify Himself to step into that role—it was presented to Him. Remember our contrast with Helel, who did glorify and exalt himself in his determination to ascend to a position to which he had not been appointed.
But getting back to Christ becoming High Priest, verse 8 says that it was through His sufferings that He was made complete and fully ready to be the sort of Priest that is needed for this New Covenant.
Notice that His role has not been static but has grown—it has branched out—as God’s plan unfolded. For instance, the Word is not spoken of as always having been a Son, but at some point, God said, “…this day I have begotten You,” and He became the Son.
Similarly, Hebrews 5:9 says that He became the author of eternal salvation, yet that was after He fulfilled the work the Father had given Him to do. There were certain things that the Most High and the Word predetermined must take place. The Word, of course, has always been without flaw, but the plan required that He “learn obedience” through suffering, which is another way of describing His expanding role.
When verse 9 says that He was “perfected,” it does not imply that He was ever morally imperfect. The word “perfect” has the meaning of “complete.” He had to be completed through living a sinless human life, and undergoing all that we do, so that He could become our Advocate and Intercessor.
Similarly, in saying that "He learned obedience," the author does not mean that He was in any way disobedient before. Jesus said clearly and truthfully that He always did what pleased the Father (John 8:29). He obeyed God in every possible way. Learning obedience means that Jesus learned what it means to obey even while suffering. He had to learn fully what it was like to continue to submit to God's will even while undergoing unfair, unjust, and intense pain and hardship.
He became personally acquainted with sufferings to a degree that none of His spiritual brothers and sisters ever will. But in doing so, He became perfect for the job of High Priest because He now has the experience to build and maintain the bridge between God and suffering man. He can now intercede before the Father on our behalf with personal understanding. By knowing what it is like for a human being to follow the will of God even under the greatest trials of heart and body, He personifies the voice of experience before God's throne and is thus able to mediate for us. He is perfect for the job.
