Albert Barnes Commentary Malachi 2:4

"And ye shall know that I have sent this commandment unto you, that my covenant may be with Levi, saith Jehovah of hosts." — Malachi 2:4 (ASV)

And you shall know that I have sent this commandment to you: this, which He had just spoken. Those who do not believe God when He threatens, know that He is in earnest and not to be trifled with, through His punishment. That My covenant might be with Levi. God willed to punish those who at that time rebelled against Him, so that He might spare those who would come after them. He disciplined the fathers, who showed their contempt for Him, so that their sons, taking warning from this, might not be cut off. He continues to say what the covenant was, which He still willed to continue, if they would repent.