What does Ephesians 4:21 mean?

Paul has just reminded the Ephesians about the sin and struggle which unbelievers carry. He has made a strong contrast between a life of sin and the way Christian believers are called to walk in the world. Since Paul has personal experience with this church, he knows they have been taught the truth of Christian living.

Paul's statement here is instructive. He clarifies that this truth—a changed life, with freedom from the power of sin—is for those who have heard the gospel and come to know Christ. As is true in any congregation, it is possible some will be there who do not know Christ. Paul seeks to make clear his words are specifically for believers. Simply trying to live well is not the point. Paul's remark here ties a person's ability to live distinctly from the sinful world to their salvation in Christ.

Paul further reminds his readers of Jesus' status as the one and only "truth" (John 14:6). In this letter, Paul called the gospel the "word of truth" (Ephesians 1:13), and referred to "speaking the truth in love" (Ephesians 4:15). Later, he will command believers to "speak the truth with his neighbor" (Ephesians 4:25), and to fasten on the "belt of truth" (Ephesians 6:14). Paul associated Jesus alone, not the philosophies of unbelievers, with truth. Though non-believers often seek truth through philosophy or other human means, ultimate truth is found only in Jesus, not the gods and goddesses of human culture.