Here the apostle shows the position that all followers of Jesus have before God. For Jesus himself said once: While you have the light, believe in the light, that you may become sons of light
(John 12:36; see also John 12:35b and Matthew 5:14–16).
It is remarkable that it is added, children of the day.
That is more than a repeat of children of the light.
It also indicates that they will one day partake in the glory of the great day which appears when their Lord Jesus returns. And that special day already lights up their everyday life now. True followers of Jesus live focused on that great day which will surely come.
Telling in this verse is the unbridgeable distance between light and darkness, between day and night. Whoever acknowledges Jesus as his Lord indeed lives in this world but is not of this world.
5 For you are all children of light, children of the day. We are not of the night or of the darkness.