'Sound' in the Bible

holding fast to the faithful message according to the teaching, in order that he may be able both to exhort with sound instruction and to reprove those who speak against [it].

This testimony is true, for which reason reprove them severely, in order that they may be sound in the faith,

But you, speak [the things] which are fitting for sound instruction.

Older men are to be temperate, worthy of respect, self-controlled, sound in faith, in love, [and] in endurance.

sound message beyond reproach, in order that {an opponent} may be put to shame, [because he] has nothing bad to say concerning us.