In doing so (Mal 2:6) he did his duty as a priest, “for,” etc.
knowledge - of the law, its doctrines, and positive and negative precepts (Lev 10:10, Lev 10:11; Deu 24:8; Jer 18:18; Hag 2:11).
the law - that is, its true sense.
messenger of ... Lord - the interpreter of His will; compare as to the prophets, Hag 1:13. So ministers are called “ambassadors of Christ” (2Co 5:20); and the bishops of the seven churches in Revelation, “angels” or messengers (Rev 2:1, Rev 2:8, Rev 2:12, Rev 2:18; Rev 3:1, Rev 3:7, Rev 3:14; compare Gal 4:14).