Here "brothers" means fellow Christians, including both men and women. Alternate translation: "brothers and sisters" (See: [[rc://en/ta/man/translate/figs-gendernotations]])
Being among the first people to be saved is spoken of as if the Thessalonian believers were "firstfruits." This can also be stated to remove the abstract nouns "salvation," "sanctification," "belief," and "truth." Alternate translation: "to be among the first people who believe what is true, whom God has saved and set apart for himself by his Spirit" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-abstractnouns]])