Romans was written to explain the faith. The Roman church was founded by believers who had been converted at Pentacost. Paul was writing to present his case for the gospel. Declares his allegience to the gospel and to Christ. Declares himself, a citizen of Rome... the most free of all men in the known world, to be a slave of Christ.
Major themes in Romans:
Paul details out what constitutes sin, why all have sinned, why we need salvation, how that salvation is achieved, how we can grow in Christ, who is the most high God, and how we are to serve our brothers and sisters in Christ and our fellow man.
Therefore since we have been made right in God's sight by faith, we have peace with God because of what Jesus Christ our Lord has done for us (5:1)