Malachi 4:6

turn . . . heart of . . . fathers to . . . children, &c.--Explained by some, that John's preaching should restore harmony in families. But Luk 1:16-17 substitutes for "the heart of the children to the fathers," "the disobedient to the wisdom of the just," implying that the reconciliation to be effected was that between the unbelieving disobedient children and the believing ancestors, Jacob, Levi, "Moses," and "Elijah" (just mentioned) (compare Mal 1:2; Mal 2:4, Mal 2:6; Mal 3:3-4). The threat here is that, if this restoration were not effected, Messiah's coming would prove "a curse" to the "earth," not a blessing. It proved so to guilty Jerusalem and the "earth," that is, the land of Judea when it rejected Messiah at His first advent, though He brought blessings (Gen 12:3) to those who accepted Him (Joh 1:11-13). Many were delivered from the common destruction of the nation through John's preaching (Rom 9:29; Rom 11:5). It will prove so to the disobedient at His second advent, though He comes to be glorified in His saints (Th2 1:6-10). curse--Hebrew, Cherem, "a ban"; the fearful term applied by the Jews to the extermination of the guilty Canaanites. Under this ban Judea has long lain. Similar is the awful curse on all of Gentile churches who love not the Lord Jesus now (Co1 16:22). For if God spare not the natural branches, the Jews, much less will He spare unbelieving professors of the Gentiles (Rom 11:20-21).

— Jamieson-Fausset-Brown Commentary (public domain)