Paul did not want enemies of the faith to have anything bad to say about the people who were associated with the gospel message. He wanted the Christians to live holy lives and serve God, which would help put the gospel in a good light. Grace teaches us to be obedient. The message of grace leads to holy living. Gluttons need grace.
Christians ought to be among the best-behaved people on earth.
Often we are not, and for that we ought to be ashamed. Our sins invite people to malign what we preach, the church we attend, and worst of all, the Savior we worship. It leads us to pursue peace and harmony in our families, learning how to constructively work through conflict. It leads us to a job performance that is marked by diligence and humility, not by office politics and stubbornness. Our way of life is the result of God working in us, motivating us to conform to the image of Christ.