Psalm 110 is a remarkable prophecy. It's quoted frequently in the New Testament, and arguably forms the basis of the doctrine of the epistles. Peter quotes Psalm 110 in Acts 2:34–35 to show that the ascended Christ has sat down at God's right hand. Hebrews takes up that same thought, quoting Psalm 110 to show that Christ's Priesthood is linked to Melchizedek's (Hebrews 5:6). Hebrews 9:24–28 goes on to point out that Christ is going to come back for us: His seat at God's right hand is not a permanent arrangement. He is there "until I make thine enemies thy footstool" (Hebrews 1:13).
God has promised to put Christ's enemies under His feet (Psalm 110:1). But not all His enemies. We were His enemies too (Colossians 1:21; Ephesians 2:1–3), but instead of the footstool, God has chosen the throne (Ephesians 2:6; Revelation 3:21).
This is the grace of God: He takes us from under the footstool to put us on the throne.