Fear can mean a loving reverence or a real terror. The context of the passage gives some clues to the correct interpretation. Previously the word was used with regards to reverence (Ephesians 5:21, Ephesians 5:33), which is the same meaning that it has in this sentence.¹ Trembling is a word used to denote a fear that is so great it cannot be concealed.² Where fear and trembling are used together elsewhere in the New Testament (Philippians 2:12; 1 Corinthians 2:3) they denote a loyalty whose nervousness lies in the thought that a loved one might be let down.³

Paul is not encouraging slaves to melt in fear before their earthly master, but to conduct themselves in a manner that will make it clear to their master that they respect him and seek to obey him.