Eph 4:18 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.
Eph 4:19 They have become callous and have given themselves up to sensuality, greedy to practice every kind of impurity.
Eph 4:20 But that is not the way you learned Christ!—
Eph 4:21 assuming that you have heard about him and were taught in him, as the truth is in Jesus,
Gentiles. All ungodly, unregenerate pagans (cf. 1 Thess. 4:5, which defines them). the futility of their minds. First, unbelievers are intellectually unproductive. As far as spiritual and moral issues are concerned, their rational processes are distorted and inadequate, inevitably failing to produce godly understanding or moral living. Their life is empty, vain, and without meaning (cf. Rom. 1:21–28; 1 Cor. 2:14; Col. 2:18).
Eph. 4:18 alienated from the life of God. Second, unbelievers are spiritually separated from God, thus ignorant of God's truth (1 Cor. 2:14), and their willing spiritual darkness and moral blindness is the result (cf. Rom. 1:21–24; 2 Tim. 3:7). They are blind, or "hard" like a rock.
Eph. 4:19 have become callous. Third, unbelievers are morally insensitive. As they continue to sin and turn away from God, they become still more apathetic about moral and spiritual things (cf. Rom. 1:32). sensuality... impurity. Fourth, unbelievers are behaviorally depraved (cf. Rom. 1:28). As they willingly keep succumbing to sensuality and licentiousness, they increasingly lose moral restraint, especially in the area of sexual sins. Impurity is inseparable from greediness, which is a form of idolatry (Eph. 5:5; Col. 3:5). That some souls may not reach the extremes of Eph. 4:17–19 is due only to God's common grace and the restraining influence of the Holy Spirit.
Eph. 4:20–21 learned... heard... taught. Three figurative descriptions of salvation, the new birth.
Eph. 4:21 as the truth is in Jesus. The truth about salvation leads to the fullness of truth about God, man, creation, history, life, purpose, relationships, heaven, hell, judgment, and everything else that is truly important. John summed this up in 1 John 5:20.
~John MacArthur on Eph. 4