A sound mind accepts what is truly good and right and rejects what is false and wrong.
What’s the test of a true or false teacher? Jesus connects soundness with good fruit. Something is sound when it is free from defect, decay, or disease and is healthy.
Good fruit is the result of sound living – living according to moral truth and upright character. The prophet Isaiah warned against the dangers of falsehood:Woe to those who call evil good and good evil, who put darkness for light and light for darkness (Isaiah 5:20).
The fruits of falsehood produce an easy religion which takes the iron out of religion, the cross out of Christianity, and any teaching which eliminates the hard sayings of Jesus, and which push the judgments of God into the background and makes us think lightly of sin.