Albert Barnes on Hebrews 8:1

Now of the things which we have spoken. Or, "of the things of which we are speaking," (Stuart;) or, as we should say, of what is said. The Greek does not necessarily mean things that had been spoken, but may refer to all that he was saying, taking the whole subject into consideration.

This is the sum. Or, this is the principal thing; referring to what he was about to say, not what he had said. Our translators seem to have understood this as referring to a summing up, or recapitulation of what he had said--and there can be no doubt that the Greek would bear this interpretation. But another exposition has been proposed, adopted by Bloomfield, Stuart, Michaelis, and Storr, among the moderns, and found also in Sindas, Theodoret, Theophylact, and others, among the ancients. It is that which regards the word rendered sum κεφαλαιον as meaning the principal thing; the chief matter; the most important point. The reason for this interpretation is, that the apostle in fact goes into no recapitulation of what he had said, but enters on a new topic relating to the priesthood of Christ. Instead of going over what he had demonstrated, he enters on a more important point, that the priesthood of Christ is performed in heaven, and that he has entered into the true tabernacle there. All which preceded was type and shadow, this was that which the former economy had adumbrated. In the previous chapters the apostle had shown that he who sustained this office was superior in rank to the Jewish priests; that they were frail and dying, and that the office in their hands was changing from one to another, but that that of Christ was permanent and abiding. He now comes to consider the real nature of the office itself; the sacrifice which was offered; the substance of which all in the former dispensation was the type. This was the principle thing κεφαλαιον the head, the most important matter; and the consideration of this is pursued through chapters 8-10.

We have such an High Priest. That is settled; proved; indisputable. The Christian system is not destitute of that which was regarded as so essential to the old dispensation--the office of a high priest.

Who is set on the right hand of a throne, etc. He is exalted to honour and glory before God. The right hand was regarded as the place of principle honour; and when it is said that Christ is at the right hand of God, the meaning is, that he is exalted to the highest honour in the universe. Mk 16:19. Of course the language is figurative--as God has no hands literally--but the language conveys an important meaning, that he is near to God, is high in his affection and love, and is raised to the most elevated situation in heaven. See Php 2:9; Eph 1:21, Eph 1:22.

(a) "who is set" Eph 1:20