You were told before not to believe only what you saw. This was to prevent you from saying: Is this the great mystery that eye has not seen nor ear heard nor man’s heart conceived? I see the water I used to see every day; does this water in which I have often bathed without being sanctified really have the power to sanctify me? Learn from this that water does not sanctify without the Holy Spirit.
You have read that the three witnesses of baptism—the water, the blood and the Spirit—are one. This means that if you take away one of these the sacrament of baptism is not conferred. What is water without the cross of Christ? Only an ordinary element without sacramental effect. Again, without water there is no sacrament of rebirth. Unless a man is born again of water and the Spirit he cannot enter into the kingdom of God. The catechumen believes in the cross of the Lord with which he too is signed, but unless he is baptized in the name of the Father, and of the Son and of the Holy Spirit, he cannot receive the forgiveness of sins or the gift of spiritual grace.