From the Encyclical Letter of Pope Pius XI
Among the wonderful developments of sacred teaching and piety, by which the plans of the divine Wisdom are daily made clear to the Church, hardly any is more manifest than the triumphant progress made by the devotion of the most Sacred Heart of Jesus. Very often indeed, during the course of past ages, Fathers, Doctors, and Saints have celebrated our Redeemer's love: and they have said, that the wound opened in the side of Christ was the hidden fountain of all graces. Moreover, from the Middle Ages onward, when the faithful began to shew a more tender piety towards the most sacred Humanity of the Saviour, contemplative souls became accustomed to penetrate through that wound almost to the very Heart itself, wounded for the love of men. And from that time, this form of contemplation became so familiar to all persons of saintly life, that there was no country or religious order in which, during this period, witnesses to it were not to be found. Finally, during recent centuries, and most especially at that period when heretics, in the name of a false piety, strove to discourage Christians from receiving the most Holy Eucharist, the veneration of the most Sacred Heart began to be openly practised, principally through the exertions of St. John Eudes, who is by no means unworthily called the founder of the liturgical worship of the Sacred Hearts of Jesus and Mary.
But in order to establish fully and entirely the worship of the most Sacred Heart of Jesus, and to spread the same throughout the whole world, God himself chose as his instrument a most humble virgin from the order of the Visitation, St. Margaret Mary Alacoque, who even in her earliest years already had a burning love for the Sacrament of the Eucharist, and to whom Christ the Lord had very many times appeared, and was pleased to make known the riches and the desires of his divine Heart. The most famous of these apparitions was that in which Jesus revealed himself to her in prayer before the blessed Sacrament, shewed her his most Sacred Heart, and, complaining that in return for his unbounded love, he met with nothing but outrages and ingratitude from mankind, he ordered her to concern herself with the establishment of a new feast, on the Friday after the Octave of Corpus Christi, on which his Heart should be venerated with due honour, and that the insults offered him by sinners in the Sacrament of love should be expiated by worthy satisfaction. But there is no one who knoweth not how many and how great were the obstacles which the handmaid of God experienced, in carrying out the commands of Christ; but, endowed with strength by the Lord himself, and actively aided by her pious spiritual directors, who exerted themselves with an almost unbelievable zeal, up to the time of her death she never ceased faithfully to carry out the duty entrusted to her by heaven.
At length, in the year 1765, the Supreme Pontiff Clement XIII approved the Mass and Office in honour of the most Sacred Heart of Jesus; and Pius IX extended the feast to the universal Church. From then on the worship of the most Sacred Heart, like an overflowing river, washing away all obstacles, hath poured itself forth over all the earth, and, at the dawn of the new century, Leo XIII, having proclaimed a jubilee, decided to dedicate the whole human race to the most Sacred Heart. This consecration was actually carried out with solemn rites in all the churches of the Catholic world, and brought about a great increase of this devotion, leading not only nations but even private families to it, who in countless numbers dedicated themselves to the Divine Heart, and submitted themselves to its royal sway. Lastly, the Sovereign Pontiff Pius XI, in order that, by its solemnity, the feast might answer more fully to the greatly widespread devotion of the Christian people, raised the feast of the most Sacred Heart of Jesus to the rite of a double of the first class, with an octave; and moreover, that the violated rights of Christ, the supreme King and most loving Lord, might be repaired, and that the sins of the nations might be bewailed, he ordered that annually, on that same feast-day, there should be recited an expiatory form of prayer in all the churches of the Christian world.
V. But thou, O Lord, have mercy upon us.
R. Thanks be to God.