Pope Pius XI taught that devotion to the Heart of Jesus is "the summary of our religion." And in 1956 Pope Pius XII wrote in his encyclical on the Sacred Heart, "Consequently, the honor paid to the Scared Heart is such as to raise it to the rank—so far as external practice is concerned—of the highest expression of Christian piety. For this is the religion of Jesus which is centered on the Mediator who is man and God, and in such a way that we cannot reach the Heart of God, save through the Heart of Christ."
The devotion to the Sacred Heart calls for reparation of sin, and the devotion should lead us to a deeper understanding of His infinite love and mercy for us. Our Lord told St. Faustina, "My daughter, know that My Heart is mercy itself. From this sea of mercy, graces flow out upon the whole world. No soul that has approached Me has ever gone away unconsoled" (Diary, 1777).
And yet, when we look at the Image of the Merciful Savior, we see rays of Blood and Water emanating from the area of His pierced Heart. The rays are emanating outward—they are going out to a hurting world. That is perhaps one of the differences; the Sacred Heart enables us to get a deeper understanding of the infinite mercy and calls us to reparation, yet the Divine Mercy now calls us to live that message to a hurting world.
Our Lord asked St. Margaret Mary that the image of His Sacred Heart be honored and venerated by all the faithful. In His first apparition to her in 1673, He said, "My Divine Heart is so passionately in love with men that it can no longer withhold the flames of that burning love." And in a later apparition St. Margaret Mary wrote, "It must be honored under the symbol of this heart of flesh, whose image He wished to be publicly exposed....Wherever this sacred image would be exposed for public veneration He would pour out His graces & blessings." [Sorce]