en_tn_lite_do_not_use/2th/02/15.md

So then, brothers, stand firm

Paul exhorts the believers to hold fast to their faith in Jesus.

hold tightly to the traditions

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. Alternate translation: "remember the traditions" or "believe the truths" (See: rc://en/ta/man/jit/figs-metaphor)

you were taught

This can be stated in active form. Alternate translation: "we have taught you" (See: rc://en/ta/man/jit/figs-activepassive)

whether by word or by our letter

"By word" here is a synecdoche for "by instructions" or "by teachings." You can make clear the implicit information. Alternate translation: "whether by what we taught you in person or by what we wrote to you in a letter" (See: rc://en/ta/man/jit/figs-explicit and rc://en/ta/man/jit/figs-synecdoche)