As the inspired apostle continues to exhort us to live in the world distinctly as Christians regardless of persecution, he now encourages us to recognize the will of God and to walk the pilgrim path with a good conscience if we do suffer for the Faith. As we confess Christ in the world by word and life, a good conscience, that is, a Christ-centered knowledge and hearty trust, gives the believer great confidence to not compromise. Peter would have us continue to witness and live as those who have nothing to hide and no shame to run from. If the world accuses us, let it be because of our good conversation [lifestyle]. If we are to suffer, let it be because of our witness to Christ as the only Lord and Savior.
This is the condition that the apostle lays out: if it is God's will that we suffer, it is better to suffer for well doing than for evil doing, but either way, it is God's will. The Apostle Paul knew he suffered for righteousness' sake and that gave him a holy boldness to stand before the greatest of the civil powers on earth at that time. Likewise, let us suffer for righteousness' sake even as our Savior suffered rather than suffering as an evil doer. That is, let suffering be Christ-centered.
In verse 18, the text says that the Savior suffered once. Positively, it means that He did, in fact, die for sins. If you are in Christ you are freed from the guilt of sins. Negatively, it means that it is never to be repeated or augmented. If you are in Christ, His sacrifice was sufficient and the life you now live ought to be one of thankful obedience, not servile striving to earn what He has graciously offered. He also suffered for sins vicariously and All of His obedience, all of His suffering is imputed to true believers graciously through faith. "For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him." (II Cor 5:21) Yet, in all of this glorious truth and as we praise the one sacrifice for sins, it is all for nothing without the last phrase of our text. Christ died once, the just for the unjust, but He was also quickened – made alive – by the Holy Spirit.
All of this glorious truth should give you confidence regardless of suffering. And it should give you confidence in living the truth in the world because your trust and confidence is entirely in the person and work of the only Lord and Savior Jesus Christ who died once, the just for the unjust.