From henceforth expecting - Or waiting. He waits there until this shall be accomplished according to the promise made to him that all things shall be subdued under him; see the notes on 1-Corinthians 15:25-27.
Till his enemies - There is an allusion here to Psalm 110:1, where it is said, "The Lord said unto my Lord, Sit thou at my right hand until I make thine enemies thy footstool." The enemies of the Redeemer are Satan, the wicked of the earth, and all the evil passions of the heart. The idea is, that all things are yet to be made subject to his will - either by a cheerful and cordial submission to his authority, or by being crushed beneath his power. The Redeemer, having performed his great work of redemption by giving himself as a sacrifice on the cross, is represented now as calmly waiting until this glorious triumph is achieved, and this promise is fulfilled. We are not to suppose that he is inactive, or that he takes no share in the agency by which this is to be done. but the meaning is, that he looks to the certain fulfillment of the promise.
His footstool - That is, they shall be thoroughly and completely subdued. The same idea is expressed in 1-Corinthians 15:25, by saying that all his enemies shall be put under his feet. The language arose from the custom of conquerors in putting their feet on the necks of their enemies, as a symbol of subjection; see Joshua 10:24; notes, Isaiah 26:5-6.

Till his enemies be made his footstool - Till all that oppose his high priesthood and sacrificial offering shall be defeated, routed, and confounded; and acknowledge, in their punishment, the supremacy of his power as universal and eternal King, who refused to receive him as their atoning and sanctifying Priest. There is also an oblique reference here to the destruction of the Jews, which was then at hand; for Christ was about to take away the second with an overwhelming flood of desolations.

(4) From henceforth expecting till his enemies be made his footstool.
(4) He prevents a private objection, that is, that yet nonetheless we are subject to sin and death, to which the apostle answers, that the full effect of Christ's power has not yet shown itself, but shall eventually appear when he will at once put to flight all his enemies, with whom we still struggle.

From henceforth expecting,.... According to God's promise and declaration to him, Psalm 110:1.
Till his enemies be made his footstool; see Gill on Hebrews 1:13.

expecting--"waiting." Awaiting the execution of His Father's will, that all His foes should be subjected to Him. The Son waits till the Father shall "send Him forth to triumph over all His foes." He is now sitting at rest (Hebrews 10:12), invisibly reigning, and having His foes virtually, by right of His death, subject to Him. His present sitting on the unseen throne is a necessary preliminary to His coming forth to subject His foes openly. He shall then come forth to a visibly manifested kingdom and conquest over His foes. Thus He fulfils Psalm 110:1. This agrees with 1-Corinthians 15:23-28. He is, by His Spirit and His providence, now subjecting His foes to Him in part (Psalm 110:1-7). The subjection of His foes fully shall be at His second advent, and from that time to the general judgment (Revelation. 19:1-20:15); then comes the subjection of Himself as Head of the Church to the Father (the mediatorial economy ceasing when its end shall have been accomplished), that God may be all in all. Eastern conquerors used to tread on the necks of the vanquished, as Joshua did to the five kings. So Christ's total and absolute conquest at His coming is symbolized.
be made his footstool--literally, "be placed (rendered) footstool of His feet."
his enemies--Satan and Death, whose strength consists in "sin"; this being taken away (Hebrews 10:12), the power of the foes is taken away, and their destruction necessarily follows.

Psalm 110:1.