Albert Barnes on Hebrews 1:13

But to which of the angels. The apostle adduces one other proof of the exaltation of the Son of God above the angels. He asks where there is an instance in which God had addressed any one of the angels, and asked him to sit at his right hand until he should subdue his enemies under him? Yet that high honour had been conferred on the Son of God; and he was therefore far exalted above them.

Sit on my right hand. Heb 1:3. This passage is taken from Ps 90:1--a Psalm that is repeatedly quoted in this epistle as referring to the Messiah, and the very passage before us is applied by the Saviour to himself, in Mt 22:43,44, and by Peter it is applied to him in Acts 2:34,35. There can be no doubt, therefore, of its applicability to the Messiah.

Until I make thine enemies thy footstool. Until I reduce them to entire subjection. A footstool is what is placed under the feet when we sit on a chair; and the phrase here means that an enemy is entirely subdued. Comp. 1Cor 15:25. The phrase, to make an enemy a footstool, is borrowed from the custom of ancient warriors, who stood on the necks of vanquished kings, on the occasion of celebrating a triumph over them, as a token of their complete prostration and subjection. Isa 10:6.--The enemies here referred to are the foes of God and of his religion; and the meaning is, that the Messiah is to be exalted until all those foes are subdued. Then he will give up the kingdom to the Father. 1Cor 15:24, seq. The exaltation of the Redeemer, to which the apostle refers here, is to the mediatorial throne. In this he is exalted far above the angels. His foes are to be subdued to him, but angels are to be employed as mere instruments in that great work.

(a) "Sit on my right hand" Ps 90:1