Paul claims to have met Christ on the road to Damascus (Acts 9:3-6) [1] and claims apostolic authority because of it (Acts 15:3-8) [2]. Furthermore, other apostles agreed that he had, or at least that he had the correct message (Gal 2:6-10) [3] and that his writings had the force of scripture (2 Peter 3:15-16) [4]. It is a letter from Peter, the head of the church in Jerusalem, claiming equating Paul's writing with "the other Scriptures", so you can't take out Paul's writing by claiming that that the original apostles did not believe what Paul wrote.

[1] https://www.biblegateway.com/passage/?search=Acts+9&version=...

[2] https://www.biblegateway.com/passage/?search=1%20Corinthians...

[3] https://www.biblegateway.com/passage/?search=Galatians%202&v...

[4] https://www.biblegateway.com/passage/?search=2%20Peter+3&ver...