"They are darkened in their understanding and separated from the life of God because of the ignorance that is in them due to the hardening of their hearts." NIV translation
Those that do not have the Holy Spirit do not understand the things of God. Their hearts are like sun baked ground where a seed cannot even be planted. God must do some tilling (preparing the soil) before they can even hear the truth.
"Having lost all sensitivity, they have given themselves over to sensuality so as to indulge in every kind of impurity, with a continual lust for more." NIV translation
Those that do not know and serve God serve their own desires and many times these are pleasures of the flesh.
After indulging in all kinds of these pleasures, they completely lose all ability to recognize sin for what it is. The sin draws them in and it becomes a continual search for new and more pleasures.
This happened here in Ephesus and it happens more and more so in our culture today.
"You, however, did not come to know Christ that way. Surely you heard of him and were taught in him in accordance with the truth that is in Jesus." NIV translation
Paul reminds the believers that they didn't come to faith in Jesus Christ to keep walking in the ways of the flesh. Instead, they were taught the truth of repentance and forgiveness that comes through Jesus Christ. There are many that want Jesus for a savior but also want the things of the world and let them be the lord of their lives. That is not the truth of the gospel because it leaves out our wanting to change (repentance) and allow Jesus to be in control of our lives.
"You were taught, with regard to your former way of life, to put off your old self, which is being corrupted by its deceitful desires;" NIV translation
Paul reminds the Ephesian believers that their faith began with a conviction in their hearts that they were sinners. This led them to repentance (putting off the old self). The old self was led by the same desires of the flesh that he spoke of in verses 17-19. These "deceitful desires" led to sin and separation from God.
"to be made new in the attitude of your minds;" NIV translation
Coming to faith in Jesus Christ involves a change of mind where we stop following our own thinking and agree with God that He is right.