And he gifted some to be apostles, prophets, evangelists, and pastors and teachers  for the effective training of God’s people – toward building up Christ’s body  until we all attain unity in faith and knowledge of the Son of God and until we reach completion – the standard of maturity that is the fullness of Christ.  He gifted these people so that we might no longer be children who are carried and tossed about by waves and every wind of teaching by the craftiness of people and their deceitful cleverness,  but instead by speaking the truth in love, we might grow toward that one perfectly whole person, who is the head – Christ.  Because of him the whole body, joined and held together by all the supporting ligaments, by the operation of each individual part, produces growth, building itself up in love.
17 Τοῦτο οὖν λέγω καὶ μαρτύρομαι ἐν κυρίῳ,
So now I must urge you in the Lord
μηκέτι ὑμᾶς περιπατεῖν,
not to walk
καθὼς καὶ τὰ ἔθνη περιπατεῖ ἐν ματαιότητι τοῦ νοὸς αὐτῶν,
as those gentiles walk with their empty heads,
18 ἐσκοτωμένοι τῇ διανοίᾳ
 blinded by their own thoughts
ὄντες ἀπηλλοτριωμένοι τῆς ζωῆς τοῦ θεοῦ διὰ τὴν ἄγνοιαν τὴν οὖσαν ἐν αὐτοῖς . . .
and alienated from the life of God because of the ignorance that is in them . . .
. . . διὰ τὴν πώρωσιν τῆς καρδίας αὐτῶν 19 οἵτινες ἀπηλγηκότες ἑαυτοὺς παρέδωκαν τῇ ἀσελγείᾳ εἰς ἐργασίαν ἀκαθαρσίας πάσης ἐν πλεονεξίᾳ.
. . . and their calloused hearts.  These people, having lost all feeling, have betrayed themselves to sensuality with all kinds of sexual perversion and greed.
This section marks another transition to exhortation. The previous section of the text focuses on the theology of the unity of the church as the body of Christ. But here with the words Τοῦτο οὖν λέγω (therefore, I say this), Paul shifts to challenge and exhort his audience to action. They’ve got two options. They follow God or the unbelievers. Paul wants them to choose God’s way, but he begins with the negative. μαρτύρομαι (I exhort/urge [you]) can mean either testify/bear witness or exhort/implore. In this case, its the latter.
The ἐν κυρίῳ (in the Lord) is an adjunct prepositional phrase. The phrase is intended to express Paul’s earnestness. By bringing Christ to his audiences’ mind, Paul brings them back to the previous verses where Christ is the standard of spiritual maturity and growth to which they are to seek. The infinitival clause, μηκέτι ὑμᾶς περιπατεῖν (not to walk), functions as the content of Paul’s exhortation. It is the complement of the verb, μαρτύρομαι (exhort/urge). The point, of course, deals with the manner of life Paul seeks for his audience to live.
Paul’s use of περιπατέω (walk) functions as a conceptual metaphor, which was very popular throughout all of Jewish Literature, especially in the Old Testament Wisdom and Prophets. Though a growing number of English translations provide the translations of live for this verb, there is good reason to believe that its basic meaning of walk should be used. For one, the metaphor in of itself, is easily understandable to the English reader without significant difficulty. Secondly, the fact that this is a conceptual metaphor (rather than a simple one used once and dropped by the way side) makes it incredibly difficult (if not impossible) to translation. The walking metaphor goes well beyond the use of this word and is implicit throughout the cotext of the next several chapters. Conceptual metaphors function at a discourse level and must be translated at a discourse level. Since some of the other parts of the metaphor would not be recognizable in English (cf. below) it becomes even more necessary for the translation to maintain the concept of walking in English. Translations that do not, such as the [T]NIV, NRSV, and NET, loose connections within this passage itself through much of chapter 5 and also important connections with the Wisdom Literature and Prophetic Literature traditions of the Old Testament where the paths of wisdom/righteousness and folly/wickedness are so visibly present.
καθὼς καὶ τὰ ἔθνη περιπατεῖ ἐν ματαιότητι τοῦ νοὸς αὐτῶν (as those gentiles walk with their empty heads) is a adverbial clause denoting the manner of walk Paul desires his audience to avoid with the prepositional phrase describing the manner in which they walk. The following two participial clauses in verse 18 (#1: ἐσκοτωμένοι τῇ διανοίᾳ & #2: ὄντες ἀπηλλοτριωμένοι τῆς ζωῆς τοῦ θεοῦ διὰ τὴν ἄγνοιαν τὴν οὖσαν ἐν αὐτοῖς, διὰ τὴν πώρωσιν τῆς καρδίας αὐτῶν) function in the same way: … the gentiles walk:
- with their empty heads (ἐν ματαιότητι τοῦ νοὸς αὐτῶν)
- blinded by their thoughts (ἐσκοτωμένοι τῇ διανοίᾳ)
- as foreigners of the life of God because of their own ignorance and the calloused hearts (ὄντες ἀπηλλοτριωμένοι τῆς ζωῆς τοῦ θεοῦ διὰ τὴν ἄγνοιαν τὴν οὖσαν ἐν αὐτοῖς διὰ τὴν πώρωσιν τῆς καρδίας αὐτῶν).
Whether there is any significant semantic difference on the basis of Paul’s use of one prepositional phrase as an adjunct and two participle clauses as adjuncts cannot be determined. All three seem quite clearly function in the same manner (no pun intended).
The first participle is a passive verb, which means that the grammatical subject is the semantic patient of the verb. Passives are a more marked construction. The agent of the clause is marked in the prepositional phrase. Since typically the agent does not need to be expressed in the passive voice, I have taken it to be slightly emphatic, which I have tried to represent in translation (blinded by their own thoughts).
The second adjunct participle is also a periphrastic construction, using the participle form of εἴμι & another participle (ὄντες ἀπηλλοτριωμένοι [lit. being alienated/separated]). This particular clause has its own adjuncts as well in the form of two prepositional phrases: διὰ. . . διὰ. . .
The first of these, διὰ τὴν ἄγνοιαν τὴν οὖσαν ἐν αὐτοῖς, (because of their ignorance that is in them), consists of the head noun ἄγνοιαν (ignorant) followed by a participle clause: τὴν οὖσαν ἐν αὐτοῖς (roughly: that is in them) in the 2nd attributive position. This is a rather unusual construction. For one, the copula is generally not even necessary here. Paul could have simply written: τὴν ἐν αὐτοῖς or even simply: τὴν ἄγνοιαν αὐτῶν to express the same meaning. These different options Paul had are roughly equivalent to “by their ignorance,” “by the ignorance in them,” and “by the ignorance that is in them.” So why would Paul choose the most drawn out construction? It seems likely that Paul is developing his extended metaphor of walking. As I argued previously (cf. Here), this grammatical construction (article noun article participle clause) is regularly and quite nearly universally used to point to a location – the ones who are in this place. If this understanding is correct, then Paul’s purpose and concern for her audience goes beyond merely the manner in which they walk, but also where they walk, figuratively speaking. This would suggest that we should maintain the idea of walking translation, instead of the simply glossing over the metaphor with the literal English “live,” represented by the [T]NIV.
The Gentiles, according to Paul, are foreigners, separated from the life of God (ὄντες ἀπηλλοτριωμένοι τῆς ζωῆς τοῦ θεοῦ). To put it simply, they are in the wrong place, walking on the path of ignorance with clouded minds (ἐσκοτωμένοι τῇ διανοίᾳ). All of this, the imagery and the syntax, it all resonates well with the wisdom tradition we find in Proverbs, particularly 1-9. For example, in Proverbs 8 we find personified wisdom calling out at the crossroads (verse 2) to everyone who is simple and foolish to join and walk with her on the path of righteousness (verse 20).
οἵτινες ἀπηλγηκότες ἑαυτοὺς παρέδωκαν τῇ ἀσελγείᾳ εἰς ἐργασίαν ἀκαθαρσίας πάσης ἐν πλεονεξίᾳ (lit. who, having lost all feeling, have betrayed/handed themselves over to sensuality with all kinds of sexual perversion and greed), is a relative clause that ends this somewhat long sentence. It connects to the αὐτῶν, to provide a more detailed description of the Gentiles and the result of their actions. The semantics of the verb, “παρέδωκαν,” requires the roles of agent, theme, & recipient . That is, for the clause to be grammatical, these three semantic slots must be filled. In this case, the agent is filled by the relative pronoun, “οἵτινες,” referring to the Gentiles. The theme slot is filled by the reflexive pronoun, “ἑαυτοὺς .” The recipient is personified sensuality, “τῇ ἀσελγείᾳ.” The language is figurative, but the meaning is clear. In their callous blindness, they’ve betrayed themselves to sensuality.
Conclusion: Thus, the Gentiles are walking with empty heads and cloudy thinking on the wrong path, the path of ignorance, separated from the life of God. Their hearts are so calloused that they indulge themselves in sensuality – all sorts of impurity and all kinds of greed.
 So now I must urge you in the Lord not to walk as those gentiles walk with their empty heads,  blinded by their own thoughts and alienated from the life of God because of their ignorance and the calloused hearts.  These Gentiles, having lost all feeling, have betrayed themselves to sensuality with all kinds of sexual perversion and greed.