Paul begins the letter of Romans by explaining exactly what his credentials are:

Paul’s Relationship with Christ (1:1a)
Paul’s calling from God (1:1)
Paul’s mission (1:5)

It is through his credentials that we see the main point of Paul's life. Everything in Paul's life revolves around the Gospel. It is because of the gospel Paul has the honor to be a bond-servant of Christ. It is for the gospel that Paul is called to be an apostle, and it is the message of the gospel that Paul is commissioned to tell the Gentiles. This is the one truth that has defined Paul’s life, and it is this passion that brings him to write Romans. The single theme of Paul's life is the single theme of Romans, which is why Luther called it "the very purest gospel."

Romans 1:1-6