Paul could be quite the lyrical writer when offering encouragement and praise to the members of the church. So what is this idea of Christians being a "letter of recommendation?" Paul meant that when people saw the Christians of the local church who had been taught in the faith by Paul and his companions, their lives reflected on him, on the quality of his teaching and formation in the faith.
Paul was a teacher and evangelist. His students lived (and traveled) throughout the Mediterranean world. They were a reflection on the value, the thoroughness, the expertise of his teaching. There was no "standardized test" for being a Christian. Paul couldn't "teach to the test." He could only teach to their hearts and hope that his teachings were written there, on their hearts, as God has written his commandments on the stones at Mt. Sinai. And Paul is obviously satisfied that he has met that test - he has touched their hearts and they provide excellent letters of recommendation for him and his ministry.
For Reflection: There is an old saying: "Everything we do teaches." All of us teach in some way - whether we teach formally in a school setting, or at home with our children, or in the marketplace. People are always observing us. What type of letter of recommendation are we for those who taught us and, ultimately, for Jesus?
Let us pray. Jesus, I thank you for my parents, teachers and pastors and all who taught me to know you, the living and true God. I strive to be a good letter of recommendation for them and for you. May I always reflect you well in all that I do.