Jamieson, Fausset and Brown on Malachi 3:5

5. I ... come near ... to judgment--I whom ye challenged, saying, "Where is the God of judgment?" (Mal 2:17). I whom ye think far off, and to be slow in judgment, am "near," and will come as a "swift witness"; not only a judge, but also an eye-witness against sorcerers; for Mine eyes see every sin, though ye think I take no heed. Earthly judges need witnesses to enable them to decide aright: I alone need none (Psa 10:11; 73:11; 94:7, &c.).

sorcerers--a sin into which the Jews were led in connection with their foreign idolatrous wives. The Jews of Christ's time also practised sorcery (Ac 8:9; 13:6; Ga 5:20; Josephus [Antiquities, 20.6; Wars of the Jews, 2.12.23]). It shall be a characteristic of the last Antichristian confederacy, about to be consumed by the brightness of Christ's Coming (Mt 24:24; 2Th 2:9; Re 13:13, 14; 16:13, 14; also Re 9:21; 18:23; 21:8; 22:15). Romanism has practised it; an order of exorcists exists in that Church.

adulterers--(Mal 2:15, 16).

fear not me--the source of all sins.