“And they shall teach my people the difference between the holy and profane, and cause them to discern between the unclean and the clean” (Ezekiel 44:23, Geneva Bible).
This is still a vital responsibility (for teachers and disciples): “Of whom we have many things to say, which are hard to be uttered, because ye are dull of hearing. For when as concerning the time ye ought to be teachers, yet have ye need again that we teach you what are the first principles of the Word of God: and are become such as have need of milk, and not of strong meat. For everyone that useth milk [“the first principles of the Word of God”], is inexpert in the Word of righteousness: for he is a babe. But strong meat [“the Word of righteousness”] belongeth to them that are of age, which through long custom have their wits exercised, to discern both good and evil” (Hebrews 5:12-14).