2 Among the many proofs of the boundless benignity of our Redeemer, there is one that stands out conspicuously, to wit the fact that when the charity of Christian people was growing cold, the Divine Charity itself was set forth to be honored by a special worship, and the riches of its bounty was made widely manifest by that form of devotion wherein worship is given to the Most Sacred Heart of Jesus, "In whom are hid all the treasures of wisdom and knowledge" ( Coloss . ii, 3). 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, when the Jansenist heresy, the most crafty of them all, hostile to love and piety towards God, was creeping in and preaching that God was not to be loved as a father but rather to be feared as an implacable judge; 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. And indeed Our Predecessor of happy memory, Leo XIII, admiring the timely opportuneness of the devotion to the Most Sacred Heart of Jesus, said very aptly in his Encyclical Letter, "Annum Sacrum," "When in the days near her origin, the Church was oppressed under the yoke of the Caesars the Cross shown on high to the youthful Emperor was at once an omen and a cause of the victory that speedily followed. And here today another most auspicious and most divine sign is offered to our sight, to wit the most Sacred Heart of Jesus, with a Cross set above it shining with most resplendent brightness in the midst of flames. Herein must all hopes be set, from hence must the salvation of men be sought and expected."
Source: Miserentissimus Redemptor (Vatican.va)