For ye were once darkness, but are now light in the Lord. Another expressive "but." To make the contrast more emphatic, it is not said, "ye were in darkness, but are now in light;" but, "ye were darkness itself, and are now light itself," and this last is explained by the usual formula, "in the Lord."
There was a celebrated Ephesian philosopher, Alexander, who was called "The Light;" but not from that source had the light come. The idea of light-giving is also involved in their being light. "Arise, shine, for thy light is come."
Walk as children of light. Another expressive image, denoting close connection with light, as if they were actually born of it; hence their lives should be full of it. The figure connecting darkness with sin and light with purity, common to all languages, underlies the exhortation.