St. Ambrose, De mysteriis 20, trans. Deferrari (Theological and dogmatic works, FC 44 (Catholic University Press, 1963), 12). =SC 25, 113, ll. 26-27. Liturgy of the hours (Universalis):
the three witnesses in baptism – the water, the blood and the Spirit – are one. This means that if you take away one of these the sacrament 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.