'Sound' in the Bible

He must hold firmly to the trustworthy word [of God] as it was taught to him, so that he will be able both to give accurate instruction in sound [reliable, error-free] doctrine and to refute those who contradict [it by explaining their error].

This description is true. So rebuke them sharply so that they will be sound in the faith and free from doctrinal error,

But as for you, teach the things which are in agreement with sound doctrine [which produces men and women of good character whose lifestyle identifies them as true Christians].

Older men are to be temperate, dignified, sensible, sound in faith, in love, in steadfastness [Christlike in character].

sound and beyond reproach in instruction, so that the opponent [of the faith] will be shamed, having nothing bad to say about us.