Anachronisms: Concepts are discussed and heresies are argued against that could not have been issues in Paul's time.
That is, someone copied a letter but changed it to meet his needs.
Not Corroborated by Acts: Paul describes events that do not appear in Acts. 54: Paul spends two years and three months in the Ephesus (Acts 19).
Therefore, using arguments from tradition like those we used for the Gospels, there is no reason to doubt Paul's authorship. Afterward, he was martyred with Peter under Nero, on June 29th, A. They give slightly different dates, because they did not use Finegan as the source for the chronology of Paul's life. That is, did he mean the Roman province or the region where the ethnic Galations lived?
However, because of the popularity of the arguments, we must discuss why doubt is cast on some of Paul's letters. To get the complete arguments, and if you have a lot of time on your hands, read the relevant sections of Donald Guthrie, New Testament Introduction, which defends Pauline authorship and/or Raymond E. 57: Paul stands before Festus and King Agrippa II, and appeals to Caesar (Acts 24-26), using his rights as a Roman citizen.
For example, he can speak of faith as belief in Jesus, or he can speak of "the faith" as the complete tradition about Jesus, which must be handed down to the next generation.