How do we know the authority of Paul's letters?

Paul introduces himself as an apostle of Christ Jesus by the will of God, underscoring that his authority comes not from human appointment, but from divine decree. His conversion on the road to Damascus, where he was chosen by Christ to be a vessel for the Gospel, exemplifies the sovereign grace of God in selecting even the most unlikely individuals for His purposes. Therefore, the letters of Paul carry the weight of divine authority, akin to a direct revelation from God Himself, as he was appointed as the chosen instrument to spread the Gospel.