Miserentissimus Redemptor is the title of an encyclical by Pope Pius XI, issued on May 8, 1928 on reparation to the Sacred Heart. This encyclical deals with the concepts of Acts of Reparation and atonement.
Referencing Pope Leo XIII's encyclical, Annum sacrum, Pius stated, "For as in olden time when mankind came forth from Noe's ark, God set His "bow in the clouds" (Genesis ix, 13), shining as the sign of a friendly covenant; so in the most turbulent times of a more recent age, ... then the most benign Jesus showed his own most Sacred Heart to the nations lifted up as a standard of peace and charity portending no doubtful victory in the combat."
In this encyclical Pope Pius XI discussed reparation as follows:
Having discussed the need for reparation Pius established the Feast of the Sacred Heart of Jesus as a day of reparation.
In the encyclical Pope Pius XI stated that Jesus Christ had "manifested Himself" to Saint Margaret and had "promised her that all those who rendered this honor to His Sacred Heart would be endowed with an abundance of heavenly graces."