Pope Pius XII wrote HAURIETIS AQUAS (1956),
The Sacred Heart of Jesus shares in a most intimate way in the life of the Incarnate Word, and has been thus assumed as a kind of instrument of the Divinity. It is therefore beyond all doubt that, in the carrying out of works of grace and divine omnipotence, His Heart, no less than the other members of His human nature is also a legitimate symbol of that unbounded love.
Under the influence of this love, our Savior, by the outpouring of His blood, became wedded to His Church: “By love, He allowed Himself to be espoused to His Church.”(79) Hence, from the wounded Heart of the Redeemer was born the Church, the dispenser of the Blood of the Redemption–whence flows that plentiful stream of Sacramental grace from which the children of the Church drink of eternal life, as we read in the sacred liturgy: “From the pierced Heart, the Church, the Bride of Christ, is born….And He pours forth grace from His Heart.”
Concerning the meaning of this symbol, which was known even to the earliest Fathers and ecclesiastical writers, St. Thomas Aquinas, echoing something of their words, writes as follows: “From the side of Christ, there flowed water for cleansing, blood for redeeming. Hence blood is associated with the sacrament of the Eucharist, water with the sacrament of Baptism, which has its cleansing power by virtue of the blood of Christ.” (75-77)
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