4 Christ, therefore, sits at the right hand of God, the Son is on the right hand of the Father, hidden from us. Let us believe. Two things are here said: that God said, “Sit on My right hand;” and added, “until I make Your enemies Your footstool;” that is, beneath Your feet. Thou dost not see Christ sitting at the right hand of the Father: yet you can see this, how His enemies are made His footstool. While the latter is fulfilled openly, believe the former to be fulfilled secretly.
What enemies are made His footstool? Those to whom imagining vain things it is said, “Why do the heathen so furiously rage together: and why do the people imagine a vain thing?” etc....He therefore sits at the right hand of God, till His enemies be placed beneath His feet. This is going on, this is taking place: although it is accomplished by degrees, it is going on without end. For though the heathen rage, will they, taking counsel together against Christ, prevent the fulfilment of these words: “I will give you the heathen for your inheritance, and the utmost parts of the earth for your possession”?...“Their memorial is perished with a cry;” but, “The Lord shall endure for ever:” as another Psalm, but not another Spirit, says.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)