17 This I say, therefore, and testify in the Lord, that you should no longer walk as the rest of the Gentiles walk, in the futility of their mind,
Verses 17 to 32 set forth how Christians are to “walk” in the Lord.
Verses 17 to 19 show the kink in the armor for walking with the Lord— the sin capacity in the soul. These verses warn believers not to fall back into the Gentile way of thinking. Paul presented a graphic picture of where the sin of the non-Christian can lead.
This I say,
Paul did not devise the next statements; they came from divine viewpoint, not human viewpoint. Human viewpoint results in a way of life that devolves and ends in futility.
The “therefore” resumes the subject of the Christian’s walk in fellowship with the Lord (Eph 4:1).
and testify [solemnly witness] in the Lord,
Paul not only said but he also testified that Christians are to guard themselves against falling back into their old life as non-Christians.
“In the Lord” indicates that this chapter is not from Paul exclusively but from the Lord Himself; this is divine revelation. The principles presented in this section of Ephesians are from the authority of the Lord. They stand in stark contrast to the world’s viewpoint and morals.
Every Christian is in a war between his flesh and spirit.
The Christian is in a titanic tug of war against the proclivities of the flesh. The believer has two dynamics residing in him, the new and the old man (Eph 4:22, 24). Non-Christians have only one nature called the old man. As soon as a person becomes a Christian his new capacity enters a conflict with his old capacity.
Ga 5:17, For the flesh lusts against the Spirit, and the Spirit against the flesh; and these are contrary to one another, so that you do not do the things that you wish.
The objective of the believer is to starve the old man and feed the new. The new man is sinless, whereas the old man is nothing but sinful. The sin capacity will never improve. If we starve the old man, we take the focus off of our flesh. If we feed the new man, we become more mature and stronger to confront the old man.