God is my witness, whom I serve with my Spirit.
Nowadays, we use the word spirit in the same way the apostle Paul used it in today’s verse. We may watch an athlete go all out in his performance and then comment that he exhibited “spirited” play, which means that his whole being was involved in his effort. When I was in college, the “Esprit de Corps” award was given to the football player who gave the most effort on the field. That is the way in which Paul served the Lord.
Paul never served the Lord without a wholehearted commitment. In so doing, he distinguished himself from the hirelings whose labor was external and insincere (John 10:11–13). So be like Paul—give a wholehearted effort in your service to Christ.