The idea of "one baptism" (supposedly baptism "in spirit") is obviously an attack on organized religious communities. Anyone could say that they have been spirit baptized and born again... but no one would have proof of that. However, it is clear that first-century Christians had to be baptized in water as a public sign of repentance. After that step the Christian's career really began (Acts 8:36,38,39; 22:16).
Paul included water baptism (he told about BAPTISMS in plural) in a short list of basic and introductory ideas of Christianism
Heb. 6:1 Therefore, now that we have moved beyond the primary doctrine about the Christ, let us press on to maturity, not laying a foundation again, namely, repentance from dead works and faith in God, 2 the teaching on baptisms and the laying on of the hands, the resurrection of the dead and everlasting judgment. 3 And this we will do, if God indeed permits.
Actually, Paul was baptized:
Acts 9:17 So An·a·niʹas went and entered the house, and he laid his hands on him and said: “Saul, brother, the Lord Jesus, who appeared to you on the road along which you were coming, has sent me so that you may recover sight and be filled with holy spirit.” 18 And immediately, what looked like scales fell from his eyes, and he recovered his sight. He then got up and was baptized, 19 and he ate some food and gained strength.
And not only him, but even Jesus, to set the example:
Matt. 3:13 Then Jesus came from Galʹi·lee to the Jordan to John, in order to be baptized by him. 14 But the latter tried to prevent him, saying: “I am the one who needs to be baptized by you, and are you coming to me?” 15 Jesus replied to him: “Let it be this time, for in that way it is suitable for us to carry out all that is righteous.” Then he quit preventing him. 16 After being baptized, Jesus immediately came up from the water; and look! the heavens were opened up, and he saw God’s spirit descending like a dove and coming upon him. 17 Look! Also, a voice from the heavens said: “This is my Son, the beloved, whom I have approved.”