Here "brothers" stands for believers in Christ. It is implied that they were in Antioch. AT: "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. AT: "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]])