Malachi 2:4
And ye shall know that I have sent this commandment unto you, that my covenant might be with Levi, saith the LORD of hosts. And ye shall know that I have sent this commandment unto you, that my covenant might be with Levi, saith the LORD of hosts.
God sent this commandment to the priests so that His covenant might continue and stand and remain with Levi. That purpose was set forth because the covenant with Levi was a covenant of life and peace, given because Levi feared and honored and reverenced God. Levi is portrayed as truthful in speech and teaching, walking with God in peace and equity and uprightness and faithfulness, and turning many people away from sin. Phinehas's zealous stand against open wrongdoing illustrates this covenant, for which God gave him a covenant of peace and an enduring priesthood for his descendants.
Priests are therefore to honor God by preserving knowledge and teaching His law as His messengers. The command confronts priests who had departed from that way, failed to honor God, corrupted the covenant, caused others to stumble, and shown partiality in the law. Their departure included treating the law selectively, disregarding requirements that conflicted with their personal interests and earthly possessions.