Addressing the Christians in Ephesus, Paul wrote: "So this I say, and affirm together with the Lord, that you walk no longer just as the Gentiles also walk, in the futility of their mind" (Ephesians 4:17). The apostle begins by aligning his authority with that of the Lord; in other words, this is God's authoritative message. And that message is that Christian people are not to conduct themselves in the same way that people of the world (Gentiles or sinners) conduct themselves. There is to be a difference in the person who has dedicated himself or herself to following Jesus.
A further description of the Gentiles is given in Ephesians 4:18-19: "Being darkened in their understanding, excluded from the life of God because of the ignorance that is in them, because of the hardness of their heart; and they, having become callous, have given themselves over to sensuality for the practice of every kind of impurity with greediness." What a horrible way of life this is. Can we imagine being darkened in our understanding and excluded from the life of God because of the ignorance we possess? And with hardened hearts, such will only continue because we would be callous (lacking pity, mercy, or feeling), giving ourselves completely over to sensuality and the practice of every kind of impurity. Paul does not paint a pretty picture of the sinful lifestyle for his readers.
Fortunately, there is good news. Notice what we have in Ephesians 4:20-21: "But you did not learn Christ in this way, if indeed you have heard Him and have been taught in Him, just as truth is in Jesus." Paul was very much aware that the Ephesians had been introduced to the pure gospel message when he was initially with them in Acts 19. They had heard about Jesus, they had been taught in Jesus and they discovered that truth is in Jesus. For that reason, Paul could continue to say in Ephesians 4:22-23: "That, in reference to your former manner of life, you lay aside the old self, which is being corrupted in accordance with the lusts of deceit, and that you be renewed in the spirit of your mind."
Here is what Paul really wants the Ephesians, and us, to understand: there must be a noticeable difference between the life one lives prior to becoming a Christian and the life one lives after becoming a Christian. The counsel Paul is giving any Christian is that "you lay aside the old self" because "it is being corrupted in accordance with the lusts of deceit." In other words, the former lifestyle is not consistent with the Christian walk. The Christian is to focus on "being renewed in the spirit of your mind." The change in lifestyle must be visible to anyone we come in contact with. The conclusion, in Ephesians 4:24, is unmistakable: "And put on the new self, which in the likeness of God has been created in righteousness and holiness of the truth." In very simple terms, the old self is to be laid aside as the Christian takes on the characteristics of God and Christ, living a life of righteousness and holiness that is inseparably linked to the truth found in God's word. If we want to be pleasing to God, the old self must die.