Albert Barnes Commentary Malachi 3:4

"Then shall the offering of Judah and Jerusalem be pleasant unto Jehovah, as in the days of old, and as in ancient years." — Malachi 3:4 (ASV)

Then (And) shall the offering of Judah and Jerusalem – The “law,” the new revelation of God, was to go forth from Zion and the word of the Lord from Jerusalem (Isaiah 2:3). Judah and Jerusalem then are here the Christian Church. They shall be pleasant (literally sweet) unto the Lord. It is a reversal (using the very same word) of what God had said about them in the time of their religious decay: they shall not offer wine-offerings to the Lord, neither shall they be sweet unto Him (Hosea 9:4); your burnt-offerings are not acceptable, nor your sacrifices sweet unto Me (Jeremiah 6:20).

As in the days of old – before the days of degeneracy; as it stands in the ancient Liturgies: “Be pleased to look upon them (the consecrated oblations) with a favorable and serene Countenance, and to accept them, as You were pleased to accept the gifts of Your righteous Abel and the sacrifice of our patriarch Abraham, and the holy sacrifice, the spotless offering, which Your high priest Melchizedec offered to You.”

The oblation of the sacrament of the eucharist, made by the Jews who would believe in Christ (which is known to have been first instituted by Christ in the city of Jerusalem, and afterward to have been continued by His disciples (Matthew 26:29; Acts 2:42, Acts 2:46)), will be pleasing to the Lord, like the sacrifices of the patriarchs, Melchizedec, Abraham, and the holy priests under the law, such as Aaron. Indeed, the truth takes precedence over the figure and shadow; the sacrifice of the new law is more excellent and acceptable to God than all the sacrifices of the law or before the law.

With this agrees what the Lord says to the synagogue (Isaiah 1:25–26, Isaiah 1:28), “I will turn My hand upon thee, and purely purge away thy dross, and take away all thy tin; and I will restore thy judges as at the first, and thy counselors, as at the beginning: and the destruction of the transgressors, and of the sinners, shall be together, and they that forsake the Lord shall be consumed.” So now it follows.