Charles Hodge on Ephesians 5:8

V. 8. This is enforced by a reference to their conversion from a previous state of sin and misery to one of holiness and blessedness. For ye were sometime darkness. As light stands for knowledge, and as knowledge, in the scriptural sense of the word, produces holiness, and holiness happiness; so darkness stands for ignorance, such ignorance as inevitably produces sin, and sin misery. Therefore, the expression, "ye were darkness," means, ye were ignorant, polluted, and wretched. But now ye are light in the Lord, i. e. in virtue of union with the Lord, ye are enlightened, sanctified, and blessed. Walk as children of the light, i. e. as the children of holiness and truth. " Children of light," means enlightened; as ‘children of famine,’ means the ‘famished;’ see ch. 2, 2. The exhortation is that they should walk in a way consistent with their character as men illuminated and sanctified by their union with the Lord Jesus.