Jamieson, Fausset & Brown

rejoiceth in the truth--rather, "rejoiceth with the truth." Exults not at the perpetration of iniquity (unrighteousness) by others (compare Gen 9:22-23), but rejoices when the truth rejoices; sympathizes with it in its triumphs (Jo2 1:4). See the opposite (Ti2 3:8), "Resist the truth." So "the truth" and "unrighteousness" are contrasted (Rom 2:8). "The truth" is the Gospel truth, the inseparable ally of love (Eph 4:15; Jo2 1:12). The false charity which compromises "the truth" by glossing over "iniquity" or unrighteousness is thus tacitly condemned (Pro 17:15).