27 Then, too, a public act of prayer and propitiation might fittingly be arranged in every diocese and, in conjunction with it, a special course of sermons, to serve as a fervent invitation to the faithful to redouble their works of mercy and penance. By this means they may hope to propitiate Almighty God and thus obtain by their prayers that renewal of Christian life which is one of the principal aims of the coming Council. As Our Predecessor Pius XI so aptly observed: "Prayer and penance are the two potent inspirations sent to us at this time by God, that we may bring back to Him our wayward human race that wanders aimlessly without a guide. They are inspirations that will disperse and remedy the first and foremost cause of all rebellion and unrest, man's revolt against God."(25)
Source: Paenitentiam Agere (Vatican.va)