I think what Paul was trying to tell us in 1 Corinthians 9 has do with being for the purpose of becoming. The issue is not that he acted like a Jew when with Jews; it is that he functioned in a way that gave him the greatest degree of acceptance for the sake of the good news of Jesus.
The issue was not primarily Paul’s behavior as slave or free, Jew or Gentile, but the purpose to which he lived as he did so. His desire was to remove any amoral inhibitor that might prevent men and women from coming to know Jesus.
Paul was telling us that when he was with slaves, he acted like a slave so that all slaves would see that the gospel was for them just as much as it was for the free.
Paul fit in until his allegiance to Christ made him stand out. When with the weak, he lived like one who was weak so that the truly weak could see how they might find life in Jesus.
How far would Paul go? How far should we go? Should we be willing to do anything short of sinning in order to share the life-changing message of Jesus with someone? I know some who would say yes, and I know some who would say no. What do you think? What do you think Paul was thinking?