Here "brothers" stands for believers in Christ. It is implied that they were in Antioch. Alternate translation: "taught the believers at Antioch" or "were teaching the believers at Antioch" (See: [[rc://en/ta/man/translate/figs-explicit]])
This can be stated in active form. Alternate translation: "Unless someone circumcises you according to the custom of Moses, God cannot save you" or "God will not save you from your sins unless you receive circumcision according to the law of Moses" (See: [[rc://en/ta/man/translate/figs-activepassive]])