Month of the Sacred Heart: Heart of Jesus, Substantially United to the Word of God
The reasons why the Church gives the highest form of worship to the Heart of the divine Redeemer…are two in number.
The first, which applies also to the other sacred members of the Body of Jesus Christ, rests on that principle whereby we recognize that His Heart, the noblest part of human nature, is hypostatically united to the Person of the divine Word. Consequently, there must be paid to it that worship of adoration with which the Church honors the Person of the Incarnate Son of God Himself.
Christ united to His divine Person a truly human nature, individual, whole and perfect, which was conceived in the most pure womb of the Virgin Mary by the power of the Holy Ghost. Nothing, then, was wanting to the human nature which the Word of God united to Himself. Consequently, He assumed it in no diminished way, in no different sense in what concerns the spiritual and the corporeal: that is, it was endowed with intellect and will and the other internal and external faculties of perception, and likewise with the desires and all the natural impulses of the senses.
The other reason which refers in a particular manner to the Heart of the divine Redeemer, and likewise demands in a special way that the highest form of worship be paid to it, arises from the fact that His Heart, more than all the other members of His body, is the natural sign and symbol of His boundless love for the human race. "There is in the Sacred Heart," as Our predecessor of immortal memory, Leo XIII, pointed out, "the symbol and express image of the infinite love of Jesus Christ which moves us to love in return."
Hence, since there can be no doubt that Jesus Christ received a true body and had all the affections proper to the same, among which love surpassed all the rest, it is likewise beyond doubt that He was endowed with a physical heart like ours; for without this noblest part of the body the ordinary emotions of human life are impossible.
Pius XII, Encyclical Letter Haurietis Aquas, May 15, 1956.