It is because of Paul that the gospel was spread to the Greeks and the Romans. Early church tradition was unanimous that he had founded churches in cities such as Corinth and Ephesus. Paul was martyred around AD 65 under the reign of Nero. Men such as Clement (Philippians 4:3), Linus (2 Timothy 4:21), and Peter (2 Peter 3:15-16) would have known him.
We have no argument from the first century disputing Paul's existence. The church could not have reasonably fabricated such a character out of thin air. He was known and written about by the Christians of the first and early second centuries.