Occasionally you'll hear someone allege that Catholics believe that baptism is a magical rite. Below, you'll find an excellent description of baptism by Saint Ambrose as it relates to water and the Holy Spirit:
Ex Tractátu sancti Ambrósii epíscopi De mystériis (Nn. 19-21. 24. 26-28: SCh 25bis, 164-170)"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 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 baptised 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.The Syrian Naaman bathed seven times under the old law, but you were baptised in the name of the Trinity. You proclaimed your faith in the Father – recall what you did – and the Son and the Spirit. Mark the sequence of events. In proclaiming this faith you died to the world, you died to the world, you rose again to God, and, as though buried to sin, you were reborn to eternal life. Believe, then, that the water is not without effect.The paralytic at the pool was waiting for someone. Who was this if not the Lord Jesus, born of a virgin? At his coming it is not a question of a shadow healing an individual, but Truth himself healing the universe. He is the one whose coming was expected, the one of whom God the Father spoke when he said to John the Baptist: He on whom you see the Spirit coming down from heaven and resting, this is the one who baptises in the Holy Spirit. He is the one witnessed to by John: I saw the Spirit coming down from heaven as a dove and resting on him. Why did the Spirit come down as a dove if not to let you see and understand that the dove sent out by holy Noah from the ark was a figure of this dove? In this way you were to recognise a type of this sacrament.Is there any room left for doubt? The Father speaks clearly in the Gospel: This is my beloved Son, in whom I am well pleased; the Son too, above whom the Holy Spirit showed himself in the form of a dove; and also the Holy Spirit, who came down as a dove. David too speaks clearly: The voice of the Lord is above the waters; the God of glory has thundered; the Lord is above the many waters. Again, Scripture bears witness for you that fire came down from heaven in answer to Gideon’s prayers, and that when Elijah prayed, God sent fire which consumed the sacrifice.Do not consider the merits of individuals but the office of the priests. If you do not look at merits, consider the merits of Peter and also of Paul in the same way you consider the merits of Elijah; they have handed on to us this sacrament which they received from the Lord Jesus. Visible fire was sent upon them to give them faith; in us who believe an invisible fire is at work. That visible fire was a sign, our invisible fire is for our instruction. Believe then that the Lord Jesus is present when he is invoked by the prayers of the priests. He said: Where two or three are gathered, there I am also. How much more does he give his loving presence where the Church is, where the sacraments are!You went down into the water. Remember what you said: I believe in the Father and the Son and the Holy Spirit. Not: I believe in a greater, a lesser and a least. You are committed by this spoken understanding of yours to believe the same of the Son as of the Father, and the same of the Holy Spirit as of the Son, with this one exception: you proclaim that you must believe in the cross of the Lord Jesus alone."