I don't think it is credible to say that Paul "invented " Christianity , there isn't really a motive for the guy to come up with a scheme that just gets him arrested a lot.
That was good enough for Thomas Paine, Lenin, Stalin, Joan of Arc, the guy who invented Koscot Interplanetary Cosmetics, Oral Roberts, Ho Chi Minh, William Miller. They heard the "voice" or perhaps "voices" and decided to act. For every one of them who was successful, there are hundreds who were arrested and killed, beaten and forgotten. Paul believed his own bullshit: so did Mao, Joan of Arc, Lenin, the previously mentioned Simon Bar Kokhba, Moses, Ezequiel, Socrates. This is not unusual human behavior.
Paul was not after monetary gain, obviously, he was a prophet, and prophets do it to attract followers and of course, they believe that they are doing God's will. Or fulfilling some sort of destiny. You are thinking in 20th Century terms: Paul was not of this century, and neither were his motives.