Jamieson, Fausset and Brown on Ephesians 5:21

21. (Php 2:3; 1Pe 5:5.) Here he passes from our relations to God, to those which concern our fellow men.

in the fear of God--All the oldest manuscripts and authorities read, "in the fear of Christ." The believer passes from under the bondage of the law as a letter, to be "the servant of Christ" (1Co 7:22), which, through the instinct of love to Him, is really to be "the Lord's freeman"; for he is "under the law to Christ" (1Co 9:21; compare Joh 8:36). Christ, not the Father (Joh 5:22), is to be our judge. Thus reverential fear of displeasing Him is the motive for discharging our relative duties as Christians (1Co 10:22; 2Co 5:11; 1Pe 2:13).