Ephesians 4:17-24

Overview:
Paul demands (insists) that the believers don't act like they did before they were saved. An unbeliever (Gentile) doesn't understand spiritual things, their thinking was messed up. Their life was separate from God because their hearts were hardened to spiritual things. Because they lost their spiritual sensitivity, they threw in the towel and  gave up. Their lives were characterized by: lust, indulgence and greed.
Paul reminds the Ephesians that that was not their spiritual history - they didn't grow up that way. You were taught to put off the old self which is sinful and corrupt, and put on the new. You must change/renew your thinking so you can put on the new self which is created to be like God. You are now to be characterized by righteousness and holiness.
Application:
We (I) get so caught up in my liberties, but Paul says stop acting like an unbeliever - stop acting like you did before you came to Christ. I totally agree that when you try to see how close you can imitate the world, but still be a Christian (once saved always saved) - you lose your spiritual sensitivity. Your focus has changed. Instead of imitating Christ, we are imitating the world - trying to walk a balance between both worlds. It can't be done!! We are to put off the old and put on the new. It's like taking a bath and smelling all fresh and clean, and then going to the hamper and putting on your smelly old underwear and clothes. We wouldn't dream of doing that, but that's what we try to do in our spiritual lives. We are clean because of Christ's payment on our behalf, but instead of desiring to stay clean and pure, we dig in the stinky hamper of the world and put on what it has to offer.
"God help me to take off and keep off the old man and only choose to wear clean clothes that have been washed by You!"