Having the understanding darkened, being alienated from the life of God through the ignorance that is in them, because of the blindness of their heart.
The reason we sense that God is remote is because there is a dissimilarity between moral characters. God and man are dissimilar now. God made man in His image, but man sinned and became unlike God in his moral nature. And because he is unlike God, communion is broken. Two enemies may hate each other and be separated and apart even though they are for a moment forced to be together. There is an alienation there—and that is exactly what the Bible calls that moral incompatibility between God and man.
God is not far away in distance, but He seems to be because He is far away in character. He is unlike man because man has sinned and God is holy. The Bible has a word for this moral incompatibility, this spiritual unlikeness between man and God—alienation. AOG123
Thank You, gracious Father, that You have provided the remedy for the alienation between You and Your children. Thank You for the blood of Jesus, whereby our blindness can be lifted and we can be brought near to You. Amen. 
Ignorant of God’s Truth
The second characteristic of ungodly persons is ignorance of God’s truth. Their thinking not only is futile but spiritually uninformed. They are 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.
General education and higher learning are more widespread today than ever in history. College graduates number in the tens of millions, and our society, like ancient Greece, prides itself in its science, technology, literature, art, and other achievements of the mind. For many people, to be called ignorant is a greater offense than to be called sinful. Yet Paul’s point in this passage is that ignorance and sin are inseparable. The ungodly may be “always learning,” but they are “never able to come to the knowledge of the truth” (2 Tim. 3:7). Fallen mankind has a built–in inability to know and comprehend the things of God—the only things that ultimately are worth knowing. When men rejected God, “they became futile in their speculations, and their foolish heart was darkened” (Rom. 1:21). Intellectual futility and foolishness combine as part of sin’s penalty.
The Greek word behind being darkened is a perfect participle, indicating a continuing condition of spiritual darkness. This darkness implies both ignorance and immorality. And darkness of understanding is coupled with exclusion from the life of God (cf. John 1:5). The cause of their darkness, ignorance, and separation from God is the hardness of their heart, their willful determination to remain in sin. Because men determine to reject Him, God judicially and sovereignly determines to blind their minds, exclude them from His presence, and confirm them in their spiritual ignorance. “For even though they knew God, they did not honor Him as God, or give thanks,” Paul explains of fallen mankind. “Professing to be wise, they became fools. … Therefore God gave them over in the lusts of their hearts to impurity” (Rom. 1:21–22, 24).
Because of the hardness of their heart, the ungodly are unresponsive to truth (cf. Isa. 44:18–20; 1 Thess. 4:5). Just as a corpse cannot hear a conversation in the mortuary, the person who is spiritually “dead in [his] trespasses and sins” (Eph. 2:1) cannot hear or understand the things of God, no matter how loudly or clearly they may be declared or evidenced in his presence. Pōrōsis (hardness) carries the idea of being rock–hard. It was used by physicians to describe the calcification that forms around broken bones and becomes harder than the bone itself. It was also used of the hard formations that sometimes occur in joints and cause them to become immobile. It could therefore connote the idea of paralysis as well as of hardness. Sin has a petrifying effect, and the heart of the person who continually chooses to sin becomes hardened and paralyzed to spiritual truth, utterly insensitive to the things of God.
Leroy Auden of the University of Chicago has written, “We hide a restless lion under a cardboard box, for while we may use other terms than guilt to describe this turbulence in our souls, the fact remains that all is not right within us.” By one way or another—by psychological game playing, rationalization, self–justification, transferring the blame, or by denying sin and eliminating morality—men try futilely to get rid of the lion of guilt. But it will not go away.
Satan plays a part in the blindness of those who refuse to believe, because “the god of this world has blinded the minds of the unbelieving, that they might not see the light of the gospel of the glory of Christ, who is the image of God” (2 Cor. 4:4). They refuse to see Christ because they refuse to see God, and their refusal is readily confirmed and reinforced by the god of this world.
And when men continually persist in following their own way, they will also eventually be confirmed in their choice by the God of heaven. The Jews who heard Jesus teach and preach had the great advantage of having had God’s Word given to them through Moses, the prophets, and other Old Testament writers. They had the even greater advantage of seeing and hearing God’s own incarnate Son. But “though He had performed so many signs before them,” John tells us, “yet they were not believing in Him. … For this cause they could not believe, for Isaiah said again, ‘He has blinded their eyes, and He hardened their heart; lest they see with their eyes, and perceive with their heart, and be converted, and I heal them’ ” (John 12:37, 39–40). Because they would not believe, they could not believe. God one day says,
“Let the one who does wrong, still do wrong; and let the one who is filthy, still be filthy” (Rev. 22:11).
When men choose to petrify their hearts by constant rejection of the light (John 12:35–36), they became 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. That is the unspeakable tragedy of unbelief, the tragedy of the person who makes himself his own god.
4:18 Blind. “They live blindfold in a world of illusion” (JBP). Their understanding was darkened. First, they had a native incapacity to understand spiritual truths, and then, because of their rejection of the knowledge of the true God, they suffered blindness as a judgment from the Lord.
Ungodly. They were alienated from the life of God, or at a great distance from Him. This was brought about by their willful, deep-seated ignorance and by the hardness of their hearts. They had rejected the light of God in creation and in conscience, and had turned to idolatry. Thereafter they had plunged farther and farther from God.
18 Beyond living with a futile mind-set, Gentiles possess a darkened understanding, according to Paul’s assessment. “Understanding” translates dianoia (GK 1379) and refers to the human faculty of comprehending, reasoning, and intelligence. In other words, a dark shadow blinds unbelievers and denies them the ability to comprehend spiritual matters “because they are spiritually discerned” (1 Co 2:14). Unbelievers are spiritually befuddled.
Repeating the thought of 2:1–10, Paul asserts that unbelievers are alienated from the life of God; i.e., they do not possess the life (using zōē, GK 2437) that derives from God alone (genitive of origin) and that he gives to believers. Paul supplies two reasons for this separation from God’s life: unbelievers’ “ignorance” and the “hardening of their hearts.” Does the ignorance “in them” point to an inherent flaw—original sin? Or does it point to their disobedience? The next phrase clarifies: they brought it on themselves by hardening their hearts against the truth about God. “Hardened hearts” is a common biblical metaphor (Ex 4:21; 7:3; Ps 95:8; Mt 19:8; Mk 3:5; Ro 11:25; also common in Qumran writings). “Hardening” translates pōrōsin (GK 4801), which literally refers to a callus or hardening of the skin. It describes a spiritual insensitivity or unresponsiveness—or a lack of remorse. Unbelievers’ repetition of wrong choices produces a spiritual hardening of the heart (the center of one’s being), with the result that their consciences become ineffective in curbing sin. A hard heart cannot distinguish between good and evil. This contrasts with the believers, whose hearts God has enlightened (1:18). So Christians ought not to live like these Gentiles any longer; they have been changed.
 Tozer, A. W., & Eggert, R. (2015). Tozer on the almighty god: a 365-day devotional. Chicago, IL: Moody Publishers.
 MacArthur, J. F., Jr. (1986). Ephesians (pp. 168–170). Chicago: Moody Press.
 MacDonald, W. (1995). Believer’s Bible Commentary: Old and New Testaments. (A. Farstad, Ed.) (p. 1937). Nashville: Thomas Nelson.
 Klein, W. W. (2006). Ephesians. In T. Longman III & D. E. Garland (Eds.), The Expositor’s Bible Commentary: Ephesians–Philemon (Revised Edition) (Vol. 12, pp. 124–125). Grand Rapids, MI: Zondervan.