Paul’s theology was completely Christocentric, meaning that everything he did was centered around Christ. According to Lea, “Paul’s thought can be described as historical, functional, and dynamic…it was historical because it was grounded in the incarnation, life, death and resurrection of Jesus. (Gal. 4:4).” Underlying all the themes Paul would cover be it ethics, anthropology, soteriology, ecclesiology, or eschatology, in the end were built upon the foundation that salvation was “in Christ” and that the church is the body of Christ, and exist because believers are first “in Christ.”
Paul gets his authority as an apostle like the rest of the apostles because he saw the Lord himself and was given that authority firsthand (Gal 1:1). As for the source of his theology he was able to form deeper thought than the average Jew since he was a Pharisee, while he was also handed down other information by different people about Christ’s death, burial, and resurrection (1 Cor. 15:3-5). The best way I could describe being in Christ to a curious non-believer, would be to explain that every believer has a function in the body of Christ. When a person accepts Christ we become a part of his body, hence the body analogy Paul uses in 1 Corinthians; and being in Christ we are to live life in a more holy manner than according to flesh.