Paul was selected as the leading rabbi of his generation by Jesus. This fact was acknowledged by Peter, John and James at the Jerusalem Council in Acts 15. Peter himself confirmed that Paul's letters were scripture.

Consider also that our Lord’s patience brings salvation, just as our beloved brother Paul also wrote you with the wisdom God gave him. He writes this way in all his letters, speaking in them about such matters. Some parts of his letters are hard to understand, which ignorant and unstable people distort, as they do the rest of the Scriptures, to their own destruction. - 2 Peter 3:16.

On top of that, John lived in Ephesus for another whole generation or so after Paul was executed in 67 AD. He had plenty of time and plenty of authority to stamp out all Paul's teachings if they were false.

In every generation, from the early Church Fathers to Augustine, Aquinas, Luther, Calvin, Wesley, Edwards, Spurgeon, Lewis, Graham, and on and on great scholars of the Bible have analyzed and taught the teachings of the Apostle Paul as the deepest and most learned of all the Scriptures. That was the job God gave Paul - to take Christianity beyond where fishermen and tax collectors could take it and harmonize it with all the prior Scriptures that he had studied as a Pharisee under Gamaliel.

Everything Paul says holds together perfectly if you take the time to understand it. Every single apparent contradiction is an opportunity to increase your understanding of the deeper truths.