The extraordinary purity of the lives of the early Christians as contrasted with the lives of the pagans about them was noted, somewhat grudgingly, by such men as Pliny, who commented on it in writing to Trajan (Letters x. 96; Loeb ed., vol. 2, pp. 401-405). Compare John 12:36; 1 Thess. 5:5; James 1:17.

 That is, conduct your life (see on ch. 2:2).

The idea here seems to be that not only were they “in the light” but they were light—by virtue of their union with Christ, who is Himself the light.

 Compare Rom. 2:19; 1 John 2:11. Formerly they were sunk in darkness and practiced the abominations of darkness (cf. Eph. 2:11, 12; 4:18).

Gr. pote, “once” or “formerly.” In Old English “sometimes” had this meaning.