What the Bible says about Ethnic Relationship
(From Forerunner Commentary)
First, notice that Paul does this on the Sabbath day in a synagogue. He speaks to "brethren." In this context, brethrendoes not mean fellow church members but "fellow Israelites"—Jews, Levites, Benjamites, or whatever tribes they were from. They were brethren in terms of their ethnic relationship.
Second, he also speaks to those described as "you who fear God." This term was used of Gentiles who were proselytes of Judaism, Gentiles who had converted to Judaism. Here is a congregation in a synagogue, consisting of Jews and Gentiles already converted to Judaism—keeping the Sabbath. By the way, this took place in the environs of Galatia.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part Twenty-Four)