So if Paul fine tuned the message of Christ and simply elaborated, that’s insufficient evidence to say he corrupted the message. That’s what the OP is about.
It's if he "championed" the cause of Christ also. Without Paul would there be "saved by grace". He championed that it isn't people doing good things to earn a place in some paradise in the sky. He argued that people can never do enough to earn salvation... that Jesus accomplished everything by his sacrifice. Christ, alone, paid the price. All people have to do is accept that free gift. Then, they are supposed to obey Christ's commands.
In the beginning, wasn't Paul's letters pretty much the only things Christians had that was written down? So use Paul's ideas and to go to non-Jews and tell them they don't have get circumcised, that they don't have to abide by any dietary laws, they don't have to observe the Sabbath laws and all the rest, makes it easy to get converts to this new religion.
But, what were Christians telling people about this new religion? The main thing was that Jesus had risen from the dead and conquered Satan. That, without Jesus, Jews or Gentiles were going to hell to pay the price for their sins. But, no matter what a person did, they could now accept that Jesus already paid the penalty for them. All they had to do is believe... and, somewhere in the fine print, they obey his rules, just not the Jewish rules.
I think there is something else happening here too. There is what people think Jesus taught. That would be all the "nice" things like do unto others and turn the other cheek. There is what Paul argued for, that it is not the Law that saves people, it is faith in God and faith in Jesus. But, also, the message of the "gospel". Which was what? What is this "good news" That good news the NT is talking about is that Jesus lives. He has risen from the grave. His life's purpose was not to say nice things and hope people will do them. His purpose was to die as an atoning sacrifice for all the people of the world. Did Paul champion that message or corrupt it?
But why did Jesus have to sacrifice himself? That's most of Paul's message. If people think they can be like Jesus, and follow his teachings on peace and love, Paul says you can't. You can never be good enough. The NT and Paul teaches that there is a devil. That people are under a curse. That they can't earn salvation. And that God sent his only Son to die, so that people could gain salvation through him.
But, this is where the Baha'i teachings come in. Because, none of that, means anything if there is no devil... if there is no hell, and if Adam and Eve didn't cause God to curse humans. So all the things that Paul teaches about salvation aren't based on anything real. Christ didn't have to save people from hell and conquer Satan. There is no hell or Satan. Christ's message and Paul's are meaningless.
The Baha'is message says the opposite. People can earn a higher place in the next spiritual world by doing good. They don't have to worry about a hell nor a devil. Did Jesus conquer death? Not to the Baha'is, he's dead. Baha'is say he lives on in spirit, but not in any physical body. But, who doesn't do that? Everybody lives on according to the Baha'is. So what is real or special about Christ's message or Paul's? Did Paul lie? Sure he did. Everybody in the New Testament is a liar, 'cause they teach Jesus rose from the dead. All of Christianity is a lie... if the Baha'i Faith is true. That's why, I'm saying, for a more realistic Baha'is view, I have to agree with Trailblazer. She don't sugarcoat it.