The MOST Theological Collection: Outline of Christology
"XXV. The Sacred Heart of Jesus and Immaculate Heart of Mary"
Pius XII, in his Haurietis aquas Encyclical May 25, 1956 carefully explained that this devotion is not a peripheral thing, like devotions to various Saints. It is part of the main line of our faith, since it is really honor paid to the love of God for us as embodied in and manifested in the Heart of Jesus.
Pius XII explained that in the Heart of Jesus there is a triple love: "It is a symbol of that divine love, which He shares with the Father and the Holy Spirit but which He, the Word made flesh, alone manifests through a weak and perishable body... . It is besides, the symbol of the burning love which, infused into His soul, enriches the human will of Christ and enlightens and governs its acts by the most perfect knowledge derived both from the beatific vision and that which is directly infused. And finally - and this in a more natural and direct way - it is the symbol also of sensible love since the body of Jesus Christ, formed by the Holy Spirit in the womb of the Virgin Mary, possesses full powers of feelings and perception, in fact more so than any other human body." (Vatican Press Version, as revised by Francis Larkin, SS. CC. , 1974, § 55-57).
Since to love is to will good to another for the other's sake, then His love consists in willing the good of eternal happiness to us, and being willing to go so far as the dreadful death of the cross to make that possible for us. Thus He "proved His love" (Rom 5. 8). The love of feeling of which Pius XII speaks is the natural counterpart in the body of the love in the spiritual will. (Cf. Wm. G. Most, Our Father's Plan, Chapter 16).
This devotion is not entirely new, nor is it fundamentally based on private revelations, not even on those given to St. Margaret Mary. (§90ff. ) It is an outgrowth of devotion to Christ's Sacred Humanity. Yet, the revelations given to St. Margaret Mary gave a great impetus to it. And the Church has shown special favor to the Great Promise of the Nine First Fridays, even making special liturgical provision for the use of the Mass of the Sacred Heart on most First Fridays throughout the year.
That Nine First Friday Promise, although it is found in a private revelation, has yet received, as we said, so many marks of favor of the Church. The best interpretation of it is this: If someone once in a lifetime made the Nine First Fridays sincerely, and did not at the start or later intend to use it in presumption to safely enter on a life of sin - yet if someone out of weakness, not out of presumption, were later to fall into sin, the promise seems to assure us that such a one "will not die out of my grace."
The essence of the devotion was well explained by Pius XI in his Miserentissimus Redemptor, of May 8, 1928. He explains that there are two essential things: consecration and reparation for sin: "If the first and chief thing in consecration is the repayment of the love of the creature to the love of the Creator, the second thing at once follows from it, that, if that Uncreated Love has been neglected by forgetfulness or violated by offense, compensation should be made in some way for the injustice that has been inflicted: in common language we call this debt one of reparation." Pius XI adds ( AAS 20. 174): "Now if the soul of Christ [in Gethsemani] was made sorrowful even to death on account of our sins, which were yet to come, but which were foreseen, there is no doubt that He received some consolation from our reparation, likewise foreseen." The means of that foresight of course was the beatific vision in His human soul.
Pope Leo XIII, when he consecrated the world to the Sacred Heart (Annum sacrum, May 25, 1889. ASS 31. 649) explained well: "For we, in dedicating ourselves, not only recognize and accept His rule explicitly and freely, but we actually testify that if that which we give were ours, we would most willingly give it, and we ask Him to graciously accept from us that very thing, even though it is already His."
We already have a consecration to God through Baptism, but it pledges less, being primarily concerned with the commandments. With the Sacred Heart consecration we pledge to do much more and better.
The various externals of piety, such as statues, hymns, while to be promoted, are not the same as this essence, which lies in consecration and reparation. It is in this perspective that Pius XII called this devotion part of the main line of our faith.
The Haurietis aquas adds the following (§ 124): "Let the faithful see to it that to this devotion the Immaculate Heart of the Mother of God be closely joined. For, by God's Will, in carrying out the work of human Redemption, the Blessed Virgin Mary was inseparably linked with Christ in such a manner that our salvation sprang from the love and the sufferings of Jesus Christ to which the love and sorrows of His Mother were intimately united." In other words, she was as Pius XII said (Munificentissimus Deus, AAS 42. 768) "always sharing His lot" en even in cooperation in the Redemption, which Pius XII in the same document spoke of a work "common to the Blessed Virgin and her Son." Vatican II splendidly filled in this picture showing that from eternity before time began to eternity after the end of time, and in every one of the mysteries of His life and death, she was "always sharing His lot. (For a fill-in cf. Wm. G. Most, Our Father's Plan, pp. 221-24. The same chapter 24 also shows that devotion to her is parallel, in both consecration and reparation, to that to the Sacred Heart). What the Father in His approach to us has joined, we should not put asunder.