17 So I tell you this, and insist on it in the Lord, that you must no longer live as the Gentiles do, in the futility of their thinking. 18 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. 19 Having lost all sensitivity, they have given themselves over to sensuality so as to indulge in every kind of impurity, and they are full of greed.
Ephesians 4:17-19 NIV
17 I say therefore, and testify in the Lord, that henceforth you must not walk like other Gentiles walk, in vanity of their mind, 18 blinded in their understanding, being alienated from the life that is in God through the ignorance that is in them because of the blindness of their hearts; 19 who, being past repentance, have given themselves over to wantonness, to work all manner of uncleanness, even with greediness.
Ephesians 4:17-19 NMB
While the vast majority of bible versions speak of the hardening of their hearts, a significant minority speak of the blindness of their hearts. For the version with the blindness of their hearts I chose the New Matthew Bible (NMB) which has the following description here
The New Matthew Bible is the only ‘modern’ Bible which is not.
This is the New Testament of William Tyndale, his final revision of 1535, now gently updated. It is not a new translation. It is William Tyndale’s work made understandable for today with the light editorial touch of Ruth Magnusson Davis. (skip) … Few people are aware that the Matthew Bible formed the base of the King James Version
So now let us look closely at verse 18 in both versions
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
[They are] blinded in their understanding, being alienated from the life that is in God through the ignorance that is in them because of the blindness of their hearts
There is an implied vanity for those who believe themselves intellectual for it is their behavior which renders them darkened or blinded in their understanding. They are indeed blind or in darkness as to the effect or impact of their behavior upon their understanding of God resulting in separation or alienation from the life God had intended for them. Their ignorance was caused by the hardening or blinding of their hearts.
The hardening or blinding of their hearts was caused by behavior as already mentioned, but what behavior? Verse 19 provides the answer
they have given themselves over to sensuality so as to indulge in every kind of impurity, and they are full of greed.
[they] have given themselves over to wantonness, to work all manner of uncleanness, even with greediness.
[they] have given themselves over unto lasciviousness, to work all uncleanness with greediness.
They have lost all sensitivity (NIV) or are past repentance (NMB) or are past feeling (KJV) because of their choices and those choices have numbed them to the better things in life offered by God. The darkening or blinding of their hearts is not some judgment of God, it is simply the choices they have made and their behavior is such they can no longer see or understand the truth when it is before them. We see this all around us in the world we live in today.
[Paul] speaks of a simple and well-known fact – a fact that is seen now as well as then that the understanding becomes darkened by indulgence in sin. A man who is intemperate, has no just views of the government of the appetites. A man who is unchaste, has no perception of the loveliness of purity. A man who is avaricious or covetous, has no just views of the beauty of benevolence. A man who indulges in low vices, will weaken his mental powers, and render himself incapable of intellectual effort. Indulgence in vice destroys the intellect as well as the body, and unfits a man to appreciate the truth of a proposition in morals, or in mathematics, or the beauty of a poem, as well as the truth and beauty of religion.
Barnes’ Notes on the Bible
There are times we marvel at how someone who seems to have everything life might have to offer in terms of riches and material things along with great health will make choices that literally destroy them physically, mentally, emotionally, along with loss of family and often times even of life. They have darkened and blinded their hearts to a life God would have had them live.
Their lives become incredibly sad and tragic. Those around them continue living the same way in the same darkness of their hearts. Their hearts blind to the life and rewards God has to offer for them. Intellectually, in their foolishness, they laugh and deride those who have accepted the free Gift of Grace offered by the sacrifice of Jesus on the cross. Perhaps they can no longer imagine a God who would love them so very much.