Jamieson, Fausset and Brown on 1 Corinthians 13:6

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