**Paul says there is NOW ONE BAPTISM.**
You’re indicating that Jesus Christ’s commissions (at the end of the gospels) were short lived, as well as his instructions for rebirth to Nicodemas (there is ONE rebirth).
Many regard Paul to be the writer of Hebrews, considered by the experts (whoever thay were) to be written two years after the epistle to the Ephesians, which says “of the ‘doctrine of baptisms”: “and this we will do, if God permit”.
Remember that Paul was reminding born again believers of the gospel, otherwise he wouldn’t be saying in Heb. 6:1-3:
“Therefore LEAVING the principals of the doctrine of Christ, let us GO ON TO PERFECTION, not LAYING AGAIN the FOUNDATION of (1)repentance from dead works, (2)and of faith toward God, (3)Of the doctrine of baptisms, (4)and the laying on of hands, (5)and of resurrection of the dead, (6)and of eternal judgement. And this we will do if God permit.” (added the numbers in parentheses)
So, of the things that I listed numerically, you would only remove the plural ‘s’ from ‘baptisms’?
As to Paul being the author of Hebrews, that I do not believe. I know that most people believe that to be true, but I have a problem with that: If Paul wrote Hebrews, written To the Hebrews, he forgot about the main makeup of the Body of Christ; the Gentiles. He states clearly that there is NOW one new man, neither Jew nor Gentile, in the body of Christ. Why in the world would he write to only one part of that Body? Why would he make that differentiation if there was NOW no difference? It doesn't make sense scripturally. I understand that many regard him at the author. I also understand that all do not know. Paul signed his epistles by "the hand of Paul" or similar style. Hebrews is not signed at all.