Ephesians 4:17-18 – Now this I say and testify in the Lord, that you must no longer walk as the Gentiles do, in the futility of their minds. They are darkened in their understanding, alienated from the life of God because of the ignorance that is in them, due to their hardness of heart.
In the prior section, Paul explained the governing and working of the church as a unified body. He spoke of how pastors, apostles, teachers, etc were to preach/teach the gospel for the edification of the saints and the winning of souls. Now he discusses general and particular characteristics that should be present in the life of every believer. 
To begin with, Paul reminds the saints where they came from and he describes the ways in which sinful people (here referred to as Gentiles) live their lives. First, they live in the “futility of their minds”. This means they follow their own thoughts and ideas, living without any revelation or guidance from God. They do whatever makes them feel good.
Sinners are “darkened in their understanding”; they are void of any spiritual knowledge. They prefer to dwell in the darkness of sin rather than the light of God’s holiness.
They are also “alienated from the life of God”. There is physical life and spiritual life. Here the apostle refers to spiritual life. Sinners are spiritually dead; they have a dislike for holiness, which is the way of life that God requires. God himself is pure, righteous, holy, good and true, while those alienated from him desire the exact opposite.
Paul is very specific that the reason they are this way is because of their own hardness of heart. They willfully and persistently shut out the light that would lead them to righteousness in Christ. God makes himself known to man by his great works, by the gospel and by love, but if a sinner will not acknowledge God, they will remain in darkness.
Ephesians 4:19 - They have become callous and have given themselves up to sensuality, greedy to practice every kind of impurity.
All men are by nature corrupt and inclined to be evil. Those who are alienated from God will give free reign to self indulgence, ignoring the stings of conscience until the pain of remorse is calloused over. Once a person is past feeling any guilt or conscience, depravity knows no bounds. God, for his part, continues to call out to sinners, but because of rebellion, their hearts are hardened rather than being softened. They have become slaves to sin and to Satan.
Ephesians 4:20-21 - But that is not the way you learned Christ! – assuming that you have heard about him and were taught in him, as the truth is in Jesus,
But Christians are different because we have “learned Christ”. We have been taught the real truth by Jesus himself both in the way he lived and what he said. That truth is also revealed to us as the Holy Spirit quickens are spiritual understanding of the word through pastors and teachers. That truth has set us free from the bondage of sin, and with the help of the Holy Spirit we can live holy lives pleasing to God.
Ephesians 4:22-24 – to put off your old self which belongs to your former manner of life and is corrupt through deceitful desires and to be renewed in the spirit of your minds and to put on the new self, created after the likeness of God in true righteousness and holiness.
These verses describe three things that are to be done: Be renewed, Put off and Put on.
Be Renewed: The only way to break the chains of sin in our lives and have our natures renewed is to accept Jesus Christ as our savior. He takes our sin upon himself, setting us free. In 2 Corinthians 5:17 the scripture says, “Therefore if any man be in Christ he is a new creation: old things are passed away; behold, all things are become new”.
Put Off: We are to put off the ‘old self’ or the ‘old man’. This is a phrase used by Paul in several places, like Romans 5&6, here in Ephesians and in Colossians 3. The old man refers to the sinful and corrupt nature that we were all born with. Because Adam sinned, all people born after him were automatically born into sin. We have a corrupt nature; it is natural for us to love and indulge in sin. Here Paul notes that these sinful desires and passions are deceitful. In other words, they look good; we expect that if we indulge in them we will feel good and they will satisfy the longing of our souls, but they will not. They will only leave us feeling bitter and empty for sinful desire can never satisfy. 
As I am sure you have already noticed, our tendencies to sin are still present with us even after we are renewed or saved. God does not make us instantly holy. It is our job to partner with the Holy Spirit in what is called “progressive sanctification”. This is an ongoing process in which we choose to pursue the ways of God, thus crucifying our evil desires. We can take comfort in the fact that the Holy Spirit will carry to perfection the work begun when you became a new creature in Christ.
Put On: If you are going to “put off” the old man, you will also need to “put on” something new. What does the new creature in Christ put on? He puts on the characteristics that God possesses, such as kindness, patience and forgiveness. He walks in holiness; telling the truth, maintaining Godly relationships, encouraging others, etc. When we act and think like God, it is an outward manifestation that reflects the change that has taken place in our spiritual man. When we reflect the love and kindness of God, we are a witness to those who still walk in darkness.
Colossians 3:12-14, - Put on then, as God’s chosen ones, holy and beloved, compassionate hearts, kindness, humility, meekness, and patience… forgiving each other as the Lord has forgiven you… and above all these put on love which binds everything together in perfect harmony.
Putting off the old and putting on the new are very general concepts. In the next six verses, Paul discusses specific examples of each.
Ephesians 4:25 – Therefore, having put away falsehood, let each one of you speak the truth with his neighbor for we are members of one another.
There can be no doubt that God hates lying. One of the original 10 commands is: Exodus 20:16 - You shall not bear false witness against your neighbor. However, Paul may have something more specific in mind here. Unity of the church is the main topic of the 4th chapter, so he seems to be indicating that unity of the body is based on mutual confidence, which can only work if we always tell the truth to one another.
Ephesians 4:26-27 - Be angry and do not sin; do not let the sun go down on your anger and give no opportunity to the devil.
“Be angry and sin not” is a quote from Psalms 4:4. Anger itself is not necessarily a sin; Jesus displayed righteous anger in Mark 3:5. Keep in mind that righteous anger is anger directed at the sin, not the sinner. We need to have pity and compassion for the sinner. If we are angry at the person as well as the fault, we are sinning. This is a very difficult thing to manage. Be careful with anger! If you find a need to express it, be sure that you reconcile with the person immediately. Otherwise, Satan has an opportunity to cause division. Anger must not be allowed to fester into wrath and bitterness.
Ephesians 4:28 - Let the thief no longer steal, but rather let him labor, doing honest work with his own hands, so that he may have something to share with anyone in need.
Theft has been a part of society forever and some of the heathen really did not think it was so bad. Paul explicitly states that theft is evil. He then goes on give a very good example of putting off the old and putting on the new. Not only should the Christian stop stealing (put off), but he should do honest work and use some of that money to share with the poor (put on).
Ephesians 4:29 – Let no corrupt talk come out of your mouths, but only such as is good for building up, as fits the occasion, that it may give grace to those who hear.
The Greek word for corrupt literally means “rotten” or “putrid”. While it could mean cursing or foul language, it also means any talk that tears down, humiliates, makes fun of, intimidates, or belittles someone else. Christians should be known for building up and encouraging others, not tearing them down.
Proverbs 12:18 There is one that speaks like the piercings of a sword: but the tongue of the wise beings health.
Ephesians 4:30 – And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption.
This sentence is short in length, but long on meaning! Several things are implied by this verse:
- The Holy Spirit demonstrates personality; he is the third Person of the Trinity – a person as much as the Father and the Son.
- The Holy Spirit loves us immeasurably. You can only grieve those you love or those who love you.
- If we persist in offending the Holy Spirit, he will suspend or withdraw his influence in our lives.
- The Holy Spirit seals us. He marks or brands us as a possession of God. As discussed in chapter one, this is an assurance of our final redemption.
Ephesians 4:31-32 – Let all bitterness and wrath and anger and clamor and slander be put away from you, along with all malice. Be kind one to another, tenderhearted, forgiving one another, as God in Christ forgave you.
All the negative emotions and actions listed in these verses are a form of malice. Malice should not be found in the life of any Christian! It will not only tear apart the church, it will bring death to the individual believer. On the other hand, forgiveness will not only enforce and strengthen church unity, it will heal and bring life to the individual believer. As always, our ultimate example of forgiveness is Christ.