Is it not sinful for Paul to take pride in his work for God?

No, for he takes pride in what God accomplishes in and through him as instrument. From a human perspective, we would say that Paul had succeeded in fulfilling his task. He looks at this quite differently, however. Christ is the One who has enabled him to complete this service for God successfully. In Christ Jesus then, I have reason to be proud of my work for God (Romans 15:17). It was Jesus himself who had called him into his service at Damascus (Acts 9:4–6). It was Jesus who appeared to him at the temple to send him far away to the Gentiles (Acts 22:17–21). It is Jesus who fills him with his Spirit (Acts 9:17–19) and who opens doors (Acts 16:6–10). Jesus sometimes appears to encourage him (Acts 23:11), and Paul works for God the Lord and has Christ Jesus to back him up. He is proud of the work of his Lord.

As such, he gives all the glory to God: For I will not venture to speak of anything except what Christ has accomplished through me to bring the Gentiles to obedience – by word and deed, by the power of signs and wonders, by the power of the Spirit of God (Romans 15:18–19a). After his call, Paul began a completely different life. Suddenly it seems that he can work miracles. All at once Gentiles come back from their disobedience and estrangement to subject themselves again to the Lord. In Ephesus they even carried items of Paul’s clothes to the sick to heal them. How different his life is now compared to when he was a rabbi studying at the feet of Gamaliel. And yet none of this is the result of lengthy preparations, intensive planning, or strategies. The Spirit suddenly and uninterruptedly bears Paul on his wind and so he experiences the power of Jesus. All that he is now as an apostle is the result of grace alone. It is the gospel that makes him a prophet, and the special powers thereof transform him into a missionary (Lietaert Peerbolte ¹).

Paul’s strange construction of Romans 15:18a in the original Greek is best illustrated in the KJV: For I will not dare to speak of anything which Christ has not wrought by me. By employing the double negative, Paul probably intended to say that apart from what Christ has worked through him, there is nothing else to talk about. In other words, [i]t is no longer I who live, but Christ who lives in me (Galatians 2:20a).²