1 Timothy 6:3
3If anyone teaches a different doctrine and doesn’t consent to sound words, the words of our Lord Jesus Christ, and to the doctrine which is according to godliness,
A different doctrine is measured against the sound (healthful) words of the Lord Jesus and to the doctrine which is according to godliness. Sound words have no taint that encourages sin; they train piety. Novelty that will not consent to Christ's words is already a departure.
Grace offers a gospel that produces holiness. Teachers remain responsible to stay with Jesus' words. Wesley: if any teach otherwise than strict practical holiness in all its branches, they have left healthful doctrine.
Calvin: Paul condemns even those who do not openly oppose sound doctrine yet corrupt it by vain babbling. Words are sound because they give health. Doctrine according to godliness must instruct in the fear of God, edify faith, and train patience, humility, and love.
Whoever does not strive to teach usefully does not teach as he ought. Brilliance without profit is neither godly nor sound. The test is Christ's words and a teaching that actually forms piety.