What does it mean that Paul called the Corinthians "my brethren"?
It means the Corinthians were genuine believers in Christ, not unbelievers or outsiders. Paul addressed the whole congregation—Gentile and Jewish members alike—as fellow brethren in the Lord. Paul would not call unbelievers brethren; this confirms their identity as members of God's spiritual family through faith in Christ.
Les explains that the term "my brethren" implies the Corinthians were believers. He points out that Paul would not call these rascals my brethren, so the term shows he regarded them as fellow members of God's family. Even though they were Gentiles, and there were also many Jews in Corinth, Paul refers to all the members of the congregation as his brethren in Christ. This comes in a passage where Paul is beseeching them to speak the same thing and have no divisions, though divisions existed in that assembly.
For it hath been declared unto me of you, my brethren, by them which are of the house of Chloe, that there are contentions among you.