The writer of this epistle is showing that Christ was a high priest who conformed to the pattern of all high priests, and so he finds another distinctive feature of the high priests of old: they did not appoint themselves. The office of priest was such an elevated responsibility that no man could take it on unless he had a calling from above.
How greatly we value our calling as believers, for we are a kingdom of priests. God has called us to a task which is greater than us, and he will instruct in what we are to do and supply every needed strength to serve him. Based on his calling we may have confidence that we can carry out what he wishes us to do.
What an honour it was! To be a high priest! At any time in the early history of the Jews there was but one high priest. He was distinctive. He was to stand before God on behalf of the people. True, his sacrifices were not effective; they were symbolic of what was to come. But what an honour to be one who taught by symbols of the coming Saviour, who prophesied him. And when Christ came as the true high priest, it was an honour for him. But the question is asked, how was it an honour for Christ? Did he not always have the very maximum honour in glory, in time past, being very God and equal with the Father, and truly divine? Was he not as honourable, and possessing of honour as much as is conceivable? How does he gain honour when he becomes a priest for us? Well he does, because now it's different. As well as his divine nature, he assumes human nature, and he makes a sacrifice in the body which secures salvation for all his own. What an earned honour that is! He always had divine honour, but now there's an earned earthly honour, which is the greatest honour imaginable to human beings: to be a Saviour, a Redeemer for millions.