The word "us" is plural and refers to Paul, those with him, and all Christians, including the readers. (See: [[rc://en/ta/man/translate/figs-inclusive]])
This can be stated in active form. AT: "you encouraged believers" or "you have helped the believers" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-activepassive]])