For the priest's lips should keep knowledge. It was the priest's duty to study the Law and to teach it faithfully, as it is said of Aaron, in Ecclesiasticus 45:17, "He gave unto him his commandments, and authority in the statutes of judgments, that he should teach Jacob the testimonies, and inform Israel in his laws."
The law, here and verses 6, 8, means system of teaching, or the torah. At his month. The priest was the appointed interpreter of the Law (see Le 10:11; Deuteronomy 17:9-11; Deuteronomy 33:10; and the note on Haggai 2:11).
He is the messenger of the Lord. He announces God's will to men, explaining the Law to meet the varied circumstances which occur in daily life; he intervenes between God and man, offering man's worship to the Lord.
So Haggai (Haggai 1:13) is called "the Lord's messenger," or angel. Some see here an allusion to Malachi's own name or office (see Introduction, § II.; comp. Deuteronomy 21:5; 2 Chronicles 17:9).