The apostle began by reminding his readers how not to walk, namely as they used to walk before their conversion to Christianity.
4:17 The "therefore"in this verse is coordinate with the one in verse 1. Here we have more instruction concerning walking worthily. Paul's exhortation that follows repeats Jesus' teaching on the importance of holiness. Christians should not conduct themselves as Gentiles who do not know the Lord. Those unbelievers do not typically have a useful aim or goal in life, the idea behind "the futility of their mind [thinking]."
4:18 Here Paul traced the attitude of typical unsaved Gentiles to its source. Lack of worthy purpose rests on unclear understanding (cf. Rom. 1:21; 2 Cor. 4:4). This in turn results from separation from the life that comes from God (cf. 2:12). Separation arises from natural ignorance of God (cf. 1 Pet. 1:14). That in turn rests on insensitivity to God and His ways (cf. Rom. 1).
4:19 As a result of this condition, unsaved Gentiles typically give themselves over to lives of sensual self-indulgence (cf. Rom. 1:24-28). The Greek word aselgeia, translated "sensuality,"contains the idea of wanton violence.115
