Here "brothers" means fellow Christians, including both men and women. AT: "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 are "firstfruits." This can also be stated to remove the abstract nouns "salvation," "sanctification," "belief," and "truth." AT: "to be among the first people who believe what is true, and 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]])
Here "traditions" refers to the truths of Christ that Paul and the other apostles taught. Paul speaks of them as if his readers could hold on to them with their hands. AT: "remember the traditions" or "believe the truths" (See: [[rc://en/ta/man/translate/figs-metaphor]])
You can make clear the implicit information. AT: "whether by what we taught you in person or by what we wrote to you in a letter" (See: [[rc://en/ta/man/translate/figs-explicit]])