1 Corinthians 9:20

And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the law, as under the law, that I might gain them that are under the law (KJV)

Then: How could a Roman citizen who looked free, be a slave? The Corinthians needed Paul's explanation. When he worked with the Jews he submitted to all the Jewish laws of the Old Testament. But he did it for a purpose - that of winning them to Christ. This is the first example of ways Paul worked as a slave (Wright, p. 115).

Now: The statement "When in Rome do as the Romans" applies here. Even though secular, it shows the wisdom of getting to know people where they are by showing respect to the salient points of their culture and behaviors.