42 And see what follows: “You shall send forth Your Spirit, and they shall be made”. You shall take away their spirit, and send forth Your own: You shall take away their spirit: they shall have no spirit of their own. Are they then forsaken? “Blessed are the poor in spirit:” but they are not forsaken. They refused to have a spirit of their own: they shall have the Spirit of God. Such were our Lord's words to the future martyrs: “It is not ye that speak, but the Spirit of your Father which speaks in you.”
Attribute not your courage to yourselves. If it is yours, He says, and not Mine, it is obstinacy, not courage. “For we are His workmanship,” says the Apostle, “created unto good works.” From His Spirit we have received grace, that we may live unto righteousness: for it is He that justifies the ungodly. “You shall take away their spirit, and they shall fail; You shall send forth Your Spirit, and they shall be made: and You shall renew the face of the earth:” that is, with new men, confessing themselves to have been justified, not righteous of their own power, so that the grace of God is in them.
What then? When He has taken away our spirit, we shall be turned again to our dust, beholding to our edification our weakness, that when we receive His Spirit we may be refreshed. See what follows: “Be the glory of the Lord for ever”. Not yours, not mine, not his, or his; not for a season, but “for ever.” “The Lord shall rejoice in His works.” Not in yours, as if they were yours: because if your works are evil, it is through your iniquity; if good, it is through the grace of God. “The Lord shall rejoice in His works.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)