2 Already more than once We have endeavored, after the example of Our predecessors Innocent XII, Benedict XIII, Clement XIII, Pius VI, and Pius IX., devoutly to foster and bring out into fuller light that most excellent form of devotion which has for its object the veneration of the Sacred Heart of Jesus; this We did especially by the Decree given on June 28, 1889, by which We raised the Feast under that name to the dignity of the first class. But now We have in mind a more signal form of devotion which shall be in a manner the crowning perfection of all the honors that people have been accustomed to pay to the Sacred Heart, and which We confidently trust will be most pleasing to Jesus Christ, our Redeemer. This is not the first time, however, that the design of which We speak has been mooted. Twenty-five years ago, on the approach of the solemnities of the second centenary of the Blessed Margaret Mary Alacoque's reception of the Divine command to propagate the worship of the Sacred Heart, many letters from all parts, not merely from private persons but from Bishops also were sent to Pius IX. begging that he would consent to consecrate the whole human race to the Most Sacred Heart of Jesus. It was thought best at the time to postpone the matter in order that a well-considered decision might be arrived at. Meanwhile permission was granted to individual cities which desired it thus to consecrate themselves, and a form of consecration was drawn up. Now, for certain new and additional reasons, We consider that the plan is ripe for fulfilment.
Source: Annum Sacrum (Vatican.va)