The word "They" refers to Jewish believers who wanted all believing Jews to keep the Jewish laws and customs.
# General Information:
The words "them" and "their" refers to Jewish people who believe in Christ.
# brother
Here "brother" means "fellow believer."
# They have been told about you ... not to follow the old customs
Apparently here there are some Jews that are distorting what Paul is teaching. He does not discourage the Jews from obeying the Law of Moses. His message is that circumcision and the other customs are not necessary for Jesus to save them. You can make explicit that the leaders of the Jewish believers in Jerusalem knew that Paul was teaching God's true message. (See: [[rc://en/ta/man/translate/figs-explicit]])
# They have been told
This can be stated in active form. AT: "People have told the Jewish believers" (See: [[rc://en/ta/man/translate/figs-activepassive]])
# to abandon Moses
Here "Moses" stands for the law of Moses. AT: "to stop obeying the laws that Moses gave us" (See: [[rc://en/ta/man/translate/figs-metonymy]])
# not to follow the old customs
To obey the old customs is spoken of as if the customs were leading them and the people follow behind. AT: "not to obey the old customs" or "not to practice the old customs" (See: [[rc://en/ta/man/translate/figs-metaphor]])