The priests had been trusted with God’s sacred teaching, as given in the law of Moses. As teachers, they were responsible for guarding God’s covenant (his binding promises) with Israel (Deuteronomy 33:9–10).
messenger (Hebrew mal’ak): This word may intentionally echo the name Malachi (Hebrew mal’aki, meaning “my messenger”). In the Old Testament, this title usually describes a prophet. Here, Malachi applies it to priests, because they were responsible for explaining and applying God’s word to the people.