3 Whichever then of these is understood, this book, in its parts of fifty Psalms each, gives an answer important and very worthy of consideration. For it seems to me not without significance, that the fiftieth is of penitence, the hundredth of mercy and judgment, the hundred and fiftieth of the praise of God in His saints. For thus do we advance to an everlasting life of happiness, first by condemning our own sins, then by living aright, that, having condemned our ill life, and lived a good life, we may attain to everlasting life. Our predestination is not wrought in ourselves, but in secret with Him, in His foreknowledge. But we are called by the preaching of repentance. We are justified in the calling of mercy and fear of judgment. He fears not judgment, who has previously attained salvation. Being called, we renounce the devil by repentance, that we may not continue under his yoke: being justified, we are healed by mercy, that we may not fear judgment: being glorified, we pass into everlasting life, where we praise God without end....The verse wherewith this Psalm concludes is the voice of life everlasting.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)