11 For I certify to you, brothers, that the gospel I preached was not devised by man. -Galatians 1:11 BSB
Paul didn’t receive spiritual revelations or insights by men, but by Jesus Christ, see verse 12 and 2 Corinthians 12:7. Not only did Paul learn the gospel of Jesus Christ, some through the Apostles, see verse 18 & Galatians 2:2, and by his own studies (Paul used to be a religious leader, schooled in the Scriptures, see Philippians 3:4-6), but he also received new details of the rapture through the Holy Spirit, see 1 Thessalonians 4:13-18 & 2 Thessalonians 2:1-12. Peter recognized Paul’s letters as Scripture, see 2 Peter 3:15-16.
2 Peter 1:20-21 makes it very clear that the Scriptures only has one author, and that is the Holy Spirit who used men, the saints, to pen down the very words of God for the salvation of those who would believe in the Lord Jesus Christ by reading or hearing it (Romans 10:17).