Jesus was explaining how must can be "born again."
It requires BOTH new birth through water and new birth through Spirit.
1 Corinthians 15:46 Is talking about the resurrection of the dead. In Ro 6:3-5, Paul says that we are baptized into Jesus' death in order to be united to Him in resurrection to eternal life.
Being born again in baptism and in Spirit is what the church has taught since the beginning.
I don't understand this felt need to refute the teaching of the apostles.
Justin Martyr (100 – 165 AD) The First Apology, Chapter LXI, “Christian Baptism”
I will also relate the manner in which we dedicated ourselves to God when we had been made new through Christ; lest, if we omit this, we seem to be unfair in the explanation we are making. As many as are persuaded and believe that what we teach and say is true, and undertake to be able to live accordingly, are instructed to pray and to entreat God with fasting, for the remission of their sins that are past, we praying and fasting with them. Then
they are brought by us where there is water, and are regenerated in the same manner in which we were ourselves regenerated. For, in the name of God, the Father and Lord of the universe, and of our Savior Jesus Christ, and of the Holy Spirit, they then receive the washing with water. For Christ also said, “Except ye be born again, ye shall not enter into the kingdom of heaven.”Now, that it is impossible for those who have once been born to enter into their mothers’ wombs, is manifest to all. And how those who have sinned and repent shall escape their sins, is declared by Esaias the prophet, as I wrote above; he thus speaks: “Wash you, make you clean;...”
And for this [rite] we have learned from the apostles this reason. Since at our birth we were born without our own knowledge or choice, by our parents coming together, and were brought up in bad habits and wicked training;
in order that we may not remain the children of necessity and of ignorance, but
may become the children of choice and knowledge, and may obtain in the water the remission of sins formerly committed,
there is pronounced over him who chooses to be born again, and has repented of his sins, the name of God the Father and Lord of the universe; ...