Every pope in modern times has affirmed devotion to the Sacred Heart, an idea which when it was first introduced to the church after almost one hundred centuries seemed strange to some. It is difficult to trace the roots to the devotion, but some feel it began with the piety surrounding the wounds of Christ, of which his wounded heart always held primacy. It was His last; it was His deepest. Critics of the devotion thought that reverence to a body part, especially a human body part of Christ denied his full divinity and was akin to some practices in nature worship. Many Protestants and even Eastern Orthodox Christians avoid this devotion.
There is a long list of saints and blessed who promoted the devotion, including the Jesuit and Dominican orders themselves. Most famously perhaps, the revelations of St. Mary Mary Alacoque popularized the devotion and popes, especially since Pius XI have written about it, grounding devotion to the Sacred Heart on solid theological and ecclesial footings. Saint Pope John Paul II's writings are particularly suited for the modern reader.
At least two essential elements mark the devotion to the Sacred Heart: reparation and mercy. We cooperate with the reparation of sins by Jesus whenever we offer prayer and penance for the indifference or offenses of ourselves or others. We are also bathed in an endless font of mercy and forgiveness by the blood and water flowing in love from Christ's wounded side.