The original context: judaizers challenging sonship status of Gentiles, slaves, and women

In the Galatian context, judaizers were challenging the full sonship status of various groups: Gentiles had to proselytize and be circumcised; slaves had lesser status under Roman law and couldn't inherit; women didn't carry the family name, married outside the family, had lower access to inheritance, and didn't have the same relationship with their father that a son did. Paul is combating all of this by affirming that in Christ, all these categories are irrelevant to one's sonship status.