Befit for become, A.V.; the sound for sound, A.V. But speak thou, etc. The apostle now brings out, in full couldst with the vain talk of the heretical teachers, the solid, sober teaching of a true man of God, in harmony with the sound doctrine of the gospel of Christ.
The sound doctrine ( τῇ ὑγιαινούσῃ διδασκαλία); as in 1 Timothy 1:10 (where see note). In 1 Timothy 6:1 ἡ διδασκαλία by itself means "the Christian faith," "the doctrine of the gospel." The varying phrases, ἡ καλὴ διδασκαλία, ἡ κατ εὐσεβείαν διδασκαλία, and ἡ ὑγιαινοῦσα διδασκαλία, all mean the same thing, with varying descriptive qualifications (see 1 Timothy 6:10).
The article "the" is not required.