17 This I say therefore, and testify in the Lord, that you no longer walk as the rest of the Gentiles also walk, in the futility of their mind,
25 Therefore, putting away falsehood, speak truth each one with his neighbor. For we are members of one another.
4:17-19 Therefore, Paul exhorts the Ephesians to not walk/live as the Gentiles do, in futility/vanity of speculations not based upon God's revelation; whose understanding is darkened, and are alienated from the vine life of God, because of the ignorance that is in them. This ignorance is due to the hardening of their heart, which is the callous formed by repeated rejection of truth (see comments on Romans 9). As a result they live for the temporal and sensual gratification of this world, defiling themselves (uncleanness cuts them off from priestly service and reward). This is the state of the carnal Christian (see Corinthians). The worse news is that it doesn't satisfy, because we were made to be satisfied by God. Pleasure becomes addictive, because it is subject to the law of diminishing returns. Most of the worldlings think “if I only had X, I'd be happy.” So they spend their days pursuing X. A few attain it, and then chase after Y, because X wasn't what they thought it would be. Those who repeat the process reach Z, and start wondering about their purpose for living. Tragic.
4:20-24 The Ephesian believers in Jesus were not taught to live like pagans. The truth in Jesus is that they were to put away/off their former way of live, the stinky old man that they were. This is the “do not be conformed to this world” of Romans 12, and the “crucified with Christ” of Galatians 2:20. “Just say no” is good, but won't work in the long term unless there is an internal change of thinking and values (see comments on Romans 12:1-2 and “Killing Deadly Desires”). There must be an inner renewal of thinking, and a new set of values and feelings. Then one can put on the “fragrant new man,” who reflects the image of God in true righteousness and holiness. This should be both an individual and corporate identity.
4:25-32 The new identity in Christ should permeate one's relationships with fellow believers, especially those with whom you don't have natural affinity (and maybe even ethnic or cultural enmity, like Jews and Gentiles). Paul gives a series of instructions that follow the format of:
Verse 29 is a great verse to memorize in your favorite version. It embodies a Biblical philosophy of speech. Corrupt is used exclusively of bad fruit of bad fish, that is useless and toxic. Corrupt communication is that which tears down another to make one feel better about themselves. Our speech should serve/minister God's grace/power to those who hear us. God's grace is the power that edifies, but sometimes one needs to blast before one builds, which is why the skills of refutation, reproof, and rebuke are in the toolbox of the mature believer.