So then, brothers, stand firm, and hold the traditions which you were taught by us, whether by word, or by letter.
Hold the traditions --  See also 1 Corinthians 11:2. Paul doesn’t make a distinction between written and oral tradition. Tradition, in the scriptural sense, may be either written or oral. Until the gospels and the epistles were written the teaching of the new covenant were transmitted orally by the apostles and evangelists.