Paul Said that he stood in defense of the Gospel, Philippians 1:17. He didn't stand in defense of the law, nor did he stand in defense of some religion. If you read the epistles that Paul wrote and you come up with some doctrine like Calvinism, Catholicism or whatever it is because you have distorted what Paul has written.
Paul's main objective was the preaching of the Gospel. It was the Gospel that justifies the ungodly, Romans 4:5 and reconciles the world unto God, 2 Corinthians 5:19. Paul spent over 20 years of his life and traveled over 10,000 miles by donkey, by foot and by ship preaching the Gospel. Paul and the apostles were not taking a survey to see who had been predestinated, nor were they making plans to build elaborate cathedrals, there main goal was to preach the Gospel to as many as would hear it. It is the preaching of the Gospel that brings people to Christ, Acts 2:41.
Just like today, there was opposition to the Gospel and justification by faith. The Corinthians were a troublesome bunch. They wanted to split the church into various groups, 1 Corinthians 1:13-17. At one time Paul threatened to beat them with a rod, 1 Corinthians 4:21.
The Galatians were not much better. Paul had grounded them in the Gospel, but when he retuned they had a different Gospel, Galatians 1:6-10. The letter to the Ephesians is full of the Gospel, it is a wonderful book as is the book of Philippians. There was a problem with the church in Colossae due to heresy, Colossians 2:4-8 also 2:18. Once again Paul had to bring them back to the Gospel and justification by faith. There were apparently some problems with sexual purity in the church at Thessalonica, Thessalonians 4:1-12, that hindered Paul and the Gospel. False teachers were the problem in the book to Titus, Titus 1:10-16.
Paul was continually in defense of the Gospel. Paul would ground a church in the Gospel only to return some months later to find out that the Judaizers had moved in and had perverted the Gospel with the law. What do you stand in defense of?