Question: How do we know that everything Paul wrote is true?
Answer: Jesus told His apostles in John 16:13 that when the Spirit of truth came He would lead them into all truth. Jesus himself taught his disciples how to interpret the Old Testament (Luke 24). He was preparing them to lead the church with authority and truth.
Paul often declares himself to be an apostle of Jesus Christ, appointed by Him after the others but in no way less authoritative than them (Romans 1:1, 5; 11:13; 1 Corinthians 1:1; 4:9; 9:1, 2, 5; 15:9; 2 Corinthians 11:5; 12:11, 12; Galatians 1:1; 2:8; et al). He indicates that his teaching is to be obeyed (1 Corinthians 14:36,37; 15:1-4; et al).
Interestingly, in 2 Peter 3:15,16 Peter refers to Paul’s writings as “Scripture” (“His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction.”). And the testimony we have from Paul and Peter is that the Scriptures are God breathed (2 Timothy 3:16) and from the Holy Spirit (2 Peter 1:21).
So from a biblical perspective, Paul’s writings are Scripture that is divinely inspired and produced by the Holy Spirit to give us doctrine and training in righteousness. They are meant to be taken by us as God’s Word. Without this assurance we would struggle to really know how to discern God’s will in our lives. Our understanding of the gospel would be at stake. We need these authoritative words in order to know how to live before God.