masters according to the flesh - in contrast to your true and heavenly Master (Eph 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” (Eph 6:9).
singleness - without double-mindedness, or “eye service” (Eph 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; Mat 6:22, Mat 6:23; Luk 11:34). “Simplicity.”