Jamieson, Fausset and Brown on Ephesians 6:5

5. Servants--literally, "slaves."

masters according to the flesh--in contrast to your true and heavenly Master (Ep 6:4). A consolatory him that the mastership to which they were subject, was but for a time [Chrysostom]; and that their real liberty was still their own (1Co 7:22).

fear and trembling--not slavish terror, but (See on 1Co 2:3; 2Co 7:15) an anxious eagerness to do your duty, and a fear of displeasing, as great as is produced in the ordinary slave by "threatenings" (Ep 6:9).

singleness--without double-mindedness, or "eye service" (Ep 6:6), which seeks to please outwardly, without the sincere desire to make the master's interest at all times the first consideration (1Ch 29:17; Mt 6:22, 23; Lu 11:34). "Simplicity."