21 “Arise, O Lord, help us”. And indeed, dearly beloved, He has arisen and helped us. For when he awoke (i.e. when He arose again, and became known to the Gentiles) on the cessation of persecutions, even those who had cleaved to the earth were raised up from the earth, and on performing penance, have been restored to Christ's body, feeble and imperfect though they were: so that in them was fulfilled the text, “Your eyes did see my substance yet being imperfect; and in Your book shall they all be written.”
“Arise, O Lord, help us, and redeem us for Your Name's sake;” that is to say, freely; for Your Name's sake, not for the sake of my merits: because You have vouchsafed to do it, not because I am worthy that You should do it unto me. For this very thing, that “we have not forgotten You;” that “our heart has not gone back;” that we “have not stretched out our hands to any strange god;” how should we have been able to achieve, except with Your help? How should we have strength for it, except through Your appealing to us within, exhorting us, and not forsaking us? Whether then we suffer in tribulations, or rejoice in prosperities, redeem Thou us, not for our merits, but for Your Name's sake.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)