Out of his pierced side there came out blood and water
It was Preparation Day, and to prevent the bodies remaining on the cross during the sabbath – since that sabbath was a day of special solemnity – the Jews asked Pilate to have the legs broken and the bodies taken away. Consequently the soldiers came and broke the legs of the first man who had been crucified with him and then of the other. When they came to Jesus, they found he was already dead, and so instead of breaking his legs one of the soldiers pierced his side with a lance; and immediately there came out blood and water. This is the evidence of one who saw it – trustworthy evidence, and he knows he speaks the truth – and he gives it so that you may believe as well. Because all this happened to fulfil the words of scripture:
Not one bone of his will be broken;
and again, in another place scripture says:
They will look on the one whom they have pierced.
The New American Bible
The Catechism of the Catholic Church
1430 Jesus’ call to conversion and penance, like that of the prophets before him, does not aim first at outward works, “sackcloth and ashes,” fasting and mortification, but at the conversion of the heart, interior conversion. Without this, such penances remain sterile and false; however, interior conversion urges expression in visible signs, gestures and works of penance.
1431 Interior repentance is a radical reorientation of our whole life, a return, a conversion to God with all our heart, an end of sin, a turning away from evil, with repugnance toward the evil actions we have committed. At the same time it entails the desire and resolution to change one’s life, with hope in God’s mercy and trust in the help of his grace. This conversion of heart is accompanied by a salutary pain and sadness which the Fathers called animi cruciatus (affliction of spirit) and compunctio cordis (repentance of heart).
1432 The human heart is heavy and hardened. God must give man a new heart. Conversion is first of all a work of the grace of God who makes our hearts return to him: “Restore us to thyself, O LORD, that we may be restored!” God gives us the strength to begin anew. It is in discovering the greatness of God’s love that our heart is shaken by the horror and weight of sin and begins to fear offending God by sin and being separated from him. The human heart is converted by looking upon him whom our sins have pierced:
Let us fix our eyes on Christ’s blood and understand how precious it is to his Father, for, poured out for our salvation it has brought to the whole world the grace of repentance.
1433 Since Easter, the Holy Spirit has proved “the world wrong about sin,” i.e., proved that the world has not believed in him whom the Father has sent. But this same Spirit who brings sin to light is also the Consoler who gives the human heart grace for repentance and conversion.
On that day, you will say: I give you thanks, O LORD; though you have been angry with me, your anger has abated, and you have consoled me.
God indeed is my savior; I am confident and unafraid. My strength and my courage is the LORD, and he has been my savior.
With joy you will draw water at the fountain of salvation,
and say on that day: Give thanks to the LORD, acclaim his name; among the nations make known his deeds, proclaim how exalted is his name.
Sing praise to the LORD for his glorious achievement; let this be known throughout all the earth.
Shout with exultation, O city of Zion, for great in your midst is the Holy One of Israel!
Source: The New American Bible