The feast of the Sacred Heart was approved for specified dioceses by Clement XIII in 1765, and extended to the whole Church by Pius IX in 1856. In 1889 Pope Leo XIII elevated it to the rank of first class, and through an encyclical letter in 1899 dedicated the whole Catholic world to the Sacred Heart of Jesus. Devotion to the Sacred Heart was also an essential component of Pope John Paul II's hopes for the "new evangelization" called for by the Church.
"For evangelization today," he said, "the Heart of Christ must be recognized as the heart of the Church: It is He who calls us to conversion, to reconciliation. It is He who leads pure hearts and those hungering for justice along the way of the Beatitudes. It is He who achieves the warm communion of the members of the one Body. It is He who enables us to adhere to the Good News and to accept the promise of eternal life. It is He who sends us out on mission. The heart-to-heart with Jesus broadens the human heart on a global scale."
Here are some of the relevant documents: Leo XIII in his Encyclical Letter Annum sacrum (1889) on the consecration of mankind to the Sacred Heart; Pius XI in Caritate Christi Compulsi (On The Sacred Heart) and Miserentissimus Redemptor (On Reparation To The Sacred Heart); Pius XII in his Encyclical Letter Haurietis aquas; Paul VI in his Apostolic Letter Investigabiles divitias Christi (1965) and John Paul II in Message on the centenary of the consecration of mankind to the Sacred Heart of Jesus (1999), in L'Osservatore Romano, 12 June 1999.