The writer will say more about Melchisedec later in chapter 7 and will pick up on this quotation again and amplify the exposition of it. Melchisedec was the King of Salem in Abraham's time, a very mysterious man and a priest also, but it is the order of Melchisedec that is significant here.
To the Spirit of God, the future is no less known than the past. God’s forethought overwhelms us, and time and again God’s people have come to see that the Lord anticipated future events and prepared for them long before any human being understood what would take place. The Lord does not need our help in anything he does, and certainly does not consult human planning or wisdom. Melchisedec was made a high priest by God with the full knowledge of all that Christ would do in the future, and when the Saviour came into the world he read this Scripture given before and knew that it applied to him, and appointed him to be a priest of a different order to that of Aaron: the high priest of the New Covenant, in which he would save his people from their sins.
How is Christ a priest forever? He was only the Saviour in the world for three years plus. He always was bearer of the office of high priest, even in eternity past. Forever it was ordained that he would come and be our high priest and Saviour, and he always will be, so in the mind of God it's an eternal office that he holds. Now he’s at the right hand of the Father, but what is he doing? By his very presence he is interceding for his people, for ever, and his very presence at the right hand of the Father is an intercession.