JMNT(i)
4 Yet you yourselves, brothers (= believers), are not continuously in darkness (dimness from being in a shadow; obscurity of gloom; absence of daylight) to the end that the day may (or: would) suddenly take you down (grasp or seize you in a corresponding manner) as a thief,
5 for you see, you all are (or: exist being) sons of (= associated with and having the qualities of) Light and sons of (= associated with and having qualities of) Day! We are not (or: do not exist) of night, nor of darkness (or: we do not belong to or have the characteristics of night, nor to or of dim obscurity from shadows and gloom).