Adam Clarke on Hebrews 8:1

Of the things which we have spoken this is the sum - The word κεφαλαιον, which we translate sum, signifies the chief, the principal, or head; or, as St. Chrysostom explains it, κεφαλαιον αει το μεγιστον λεγεται, "that which is greatest is always called kephalaion," i.e. the head, or chief.

Who is set on the right hand of the throne - This is what the apostle states to be the chief or most important point of all that he had yet discussed. His sitting down at the right hand of the throne of God, proves,

1. That he is higher than all the high priests that ever existed.

2. That the sacrifice which he offered for the sins of the world was sufficient and effectual, and as such accepted by God.

3. That he has all power in the heavens and in the earth, and is able to save and defend to the uttermost all that come to God through him.

4. That he did not, like the Jewish high priest, depart out of the holy of holies, after having offered the atonement; but abides there at the throne of God, as a continual priest, in the permanent act of offering his crucified body unto God, in behalf of all the succeeding generations of mankind. It is no wonder the apostle should call this sitting down at the right hand of the throne of the Divine Majesty, the chief or head of all that he had before spoken.