Can anything be added to the relationship between Christ and the believer?
No. Nothing can be added to the relationship between Christ and the believer except faith, and faith alone in Paul's Gospel. Paul and Barnabas communicated this point by point to Jerusalem's leadership, showing it was not supplemented by additional requirements. Salvation in Paul's teaching includes justification, sanctification, forgiveness, redemption, and the indwelling Holy Spirit.
Les explains that Paul and Barnabas came from Antioch and sought to prove to the Jerusalem leadership that you cannot add anything to the relationship between Christ and the believer except faith, and faith alone in Paul's Gospel. They did not flippantly state their belief; they communicated point by point, line upon line. These revelations through Paul were so above and beyond Judaism that it took time for people to understand. Les also reminds teachers handling God's Word of the awesome responsibility, and notes Paul's Gospel is the power of God unto salvation, involving justification, sanctification, forgiveness, redemption, baptism into the Body of Christ, and the indwelling Holy Spirit.