Having a good conscience, that when they defame you as evildoers, those who revile your good conduct in Christ may be ashamed.
1 Peter 3:16
The conscience either accuses or excuses a person, acting as a source of conviction or affirmation. A good conscience doesn’t accuse a believer of sin because he is living a godly life. Instead, a good conscience affirms that everything is well, while an evil conscience points out sin.
A believer is to live with a clear conscience so that the weight of guilt won’t burden him when he faces hostile criticism. However, if he doesn’t have a passion for doing good or serving Christ, he will know the heavy weight of deserved guilt. A defiled conscience can’t be at ease or withstand the onslaught of trials. But a clear conscience will help you not to be anxious or troubled during your trials.
A Pure Conscience
and keep a good conscience so that in the thing in which you are slandered, those who revile your good behavior in Christ will be put to shame. (3:16)
The final thing that will allow believers to be secure in a hostile world is a pure conscience. The conscience is the divinely-placed internal mechanism that either accuses or excuses a person, acting as a means of conviction or affirmation. As I write elsewhere:
The conscience is the soul reflecting on itself; both the Greek word suneidēsis (conscience) and the English word “conscience” have the idea of knowing oneself. According to Roman 2:14, even those without God’s written law have an innate moral sense of right and wrong: “For when Gentiles who do not have the Law do instinctively the things of the Law, these, not having the Law, are a law to themselves.” The conscience either affirms right behavior or condemns sinful behavior.
The conscience, however, is not infallible. It is neither the voice of God, nor His moral law, as Colin G. Kruse helpfully observes:
-The conscience is not to be equated with the voice of God or even the moral law, rather it is a human faculty which adjudicates upon human action by the light of the highest standard a person perceives.
-Seeing that all of human nature has been affected by sin, both a person’s perception of the standard of action required and the function of the conscience itself (as a constituent part of human nature) are also affected by sin. For this reason conscience can never be accorded the position of ultimate judge of one’s behavior. It is possible that the conscience may excuse one for that which God will not excuse, and conversely it is equally possible that conscience may condemn a person for that which God allows. The final judgment therefore belongs only to God (cf. 1 Cor. 4:2–5). Nevertheless, to reject the voice of conscience is to court spiritual disaster (cf. 1 Tim. 1:19). We cannot reject the voice of conscience with impunity, but we can modify the highest standard to which it relates by gaining for ourselves a greater understanding of the truth. (The Second Epistle of Paul to the Corinthians, The Tyndale New Testament Commentaries [Grand Rapids: Eerdmans, 1995], 70–71)
Since the conscience holds people to their highest perceived standard, believers need to set that standard to the highest level by submitting to all of God’s Word. As they continually fill their minds with the truths of Scripture, believers clarify God’s perfect law. Their consciences will then call them to live according to that law.
The conscience functions like a skylight, not like a lamp; it does not produce its own light, but merely lets moral light in. Because of that, the Bible teaches the importance of keeping a clear or good conscience. “The goal of our instruction,” Paul wrote to Timothy, “is love from a pure heart and a good conscience and a sincere faith” (1 Tim. 1:5). A few verses later Paul stressed the importance of “keeping faith and a good conscience, which,” he warned, “some have rejected and suffered shipwreck in regard to their faith” (v. 19). A necessary qualification for deacons is that they hold “to the mystery of the faith with a clear conscience” (1 Tim. 3:9). Peter commanded believers to “keep a good conscience so that in the thing in which you are slandered, those who revile your good behavior in Christ will be put to shame” (1 Peter 3:16). Both Paul (Acts 23:1; 2 Tim. 1:3) and the writer of Hebrews (Heb. 13:18) testified that they had maintained good consciences.
At salvation, God cleanses the conscience from its lifelong accumulation of guilt, shame, and self-contempt. The writer of Hebrews wrote that “the blood of Christ, who through the eternal Spirit offered Himself without blemish to God, [will] cleanse [the] conscience from dead works to serve the living God” (Heb. 9:14). As a result, believers have their “hearts sprinkled clean from an evil conscience” (Heb. 10:22). The cleansed conscience no longer accuses because of past sins, which are pardoned (Pss. 32:5; 103:12; Prov. 28:13; Mic. 7:18–19; Col. 1:14; 2:13–14; 1 John 1:9) through the blood of Christ (Eph. 1:7; 1 John 1:7; Rev. 1:5).
Believers’ must guard the purity of their cleansed consciences, winning the battle for holiness on the inside where conscience works. Paul gained victory at that point, so that he declared to the Sanhedrin, “I have lived my life with a perfectly good conscience before God up to this day” (Acts 23:1), and to the Roman governor Felix, “I also do my best to maintain always a blameless conscience both before God and before men” (Acts 24:16). He wrote to Timothy, “I thank God, whom I serve with a clear conscience” (2 Tim. 1:3). He reminded his young protégé that “the goal of our instruction is love from a pure heart and a good conscience and a sincere faith” (1 Tim. 1:5) and exhorted him to keep “a good conscience, which some have rejected and suffered shipwreck in regard to their faith” (1 Tim. 1:19). As noted above, Paul instructed that deacons must hold “to the mystery of the faith with a clear conscience” (1 Tim. 3:9). Christians must also be careful not to cause other believers to violate their consciences (1 Cor. 8:7–13; 10:24–29). (John MacArthur, 2 Corinthians, MacArthur New Testament Commentary [Chicago: Moody, 2003], 30–32)
A good conscience is what every Christian must keep or, better, maintain.
A clear conscience allows believers to be free from any burden of guilt as they face hostility and criticism from the world (cf. Job 27:6; Rom. 14:22; 1 Tim. 3:9). An impure conscience, however, cannot be comfortable (cf. Gen. 42:21; 2 Sam. 24:10; Acts 2:37) and is unable to withstand the stress originating from difficult trials and persecutions. Regarding the thing in which they are slandered, believers ought to be able to agree with the apostle Paul, who declared, “I also do my best to maintain always a blameless conscience both before God and before men” (Acts 24:16; cf. 2 Cor. 1:12). (For a more in-depth discussion of the conscience, see John MacArthur, The Vanishing Conscience [Dallas: Word, 1994], especially chaps. 2, 3, 10, and appendixes 2 and 3.)
Slandered believers who maintain good behavior in Christ will have their consciences at rest, untroubled by guilt, and their godly lives will prove any criticisms from unbelievers to be false. Slandered (katalaleisthe) is an onomatopoetic word (one whose pronunciation suggests its meaning) that describes “evil speaking” or “verbal abuse.” Revile means “to threaten,” “to insult,” or “to mistreat.” A pure conscience can withstand and deflect whatever abusive, insulting speech the world hurls at it (cf. 1 Cor. 4:12). Those who engage in such sinful mistreatment of obedient believers (Pss. 42:10; 74:10; Matt. 27:29, 31, 41, 44; Mark 15:32; Luke 23:36; Acts 2:13), with the aim of shaming and defeating them, will themselves be put to shame (cf. Gen. 42:8–21).
Adversity is a reality and suffering a spiritual privilege for believers. If they realize “that God causes all things to work together for good to those who love God” (Rom. 8:28), they will be able to accept suffering as part of God’s plan for them and equip themselves with His securities against a hostile world. Puritan Thomas Watson wrote,
Afflictions work for good, as they make way for glory.… Not that they merit glory, but they prepare for it. As ploughing prepares the earth for a crop, so afflictions prepare and make us [ready] for glory. The painter lays his gold upon dark colours, so God first lays the dark colours of affliction, and then He lays the golden colour of glory. The vessel is first seasoned before wine is poured into it: the vessels of mercy are first seasoned with affliction, and then the wine of glory is poured in. Thus we see afflictions are not prejudicial, but beneficial, to the saints. (All Things for Good [reprint; Edinburgh: Banner of Truth, 1986], 32)
3:16 The believer must have a good conscience. If he knows he is innocent of any crime, he can go through persecution with the boldness of a lion. If he has a bad conscience, he will be plagued with feelings of guilt and will not be able to stand against the foe. Even if a believer’s life is blameless, the enemies of the gospel will still find fault with him and bring false charges against him. But when the case comes to trial, and the charges are found to be empty, the accusers will be ashamed.
 MacArthur, J. (2001). Truth for today : a daily touch of God’s grace (p. 122). Nashville, Tenn.: J. Countryman.
 MacArthur, J. F., Jr. (2004). 1 Peter (pp. 202–204). Chicago: Moody Publishers.
 MacDonald, W. (1995). Believer’s Bible Commentary: Old and New Testaments. (A. Farstad, Ed.) (p. 2271). Nashville: Thomas Nelson.