Jamieson, Fausset and Brown on 1 John 5:21

21. Affectionate parting caution.

from idols--Christians were then everywhere surrounded by idolaters, with whom it was impossible to avoid intercourse. Hence the need of being on their guard against any even indirect compromise or act of communion with idolatry. Some at Pergamos, in the region whence John wrote, fell into the snare of eating things sacrificed to idols. The moment we cease to abide "in Him that is true (by abiding) in Jesus Christ," we become part of "the world that lieth in the wicked one," given up to spiritual, if not in all places literal, idolatry (Ep 5:5; Col 3:5).