There flowed from his side water and blood. Beloved, do not pass over this mystery without thought; it has yet another hidden meaning, which I will explain to you. I said that water and blood symbolized baptism and the holy eucharist. From these two sacraments the Church is born: from baptism, the cleansing water that gives rebirth and renewal through the Holy Spirit, and from the holy eucharist. Since the symbols of baptism and the eucharist flowed from his side, it was from his side that Christ fashioned the Church, as he had fashioned Eve from the side of Adam.
From the Catecheses by Saint John Chrysostom, bishop
The Divine Mercy Novena (begin on Good Friday)