John Calvin Commentary 1 Peter 3:16

"having a good conscience; that, wherein ye are spoken against, they may be put to shame who revile your good manner of life in Christ." — 1 Peter 3:16 (ASV)

With meekness. This is a most necessary admonition, for unless our minds are imbued with meekness, contentions will immediately break forth. Meekness is set in opposition to pride and empty showiness, and also to excessive zeal. To this he justly adds fear; for where reverence for God prevails, it tames all the ferocity of our minds and will especially cause us to speak calmly of God’s mysteries. For contentious disputes arise because many think less honorably than they should of the greatness of divine wisdom and are carried away by profane audacity. If, then, we want our confession of faith to be approved by God, all boasting must be put aside, and all contention must be relinquished.

Having a good conscience. What we say without a corresponding life carries little weight; therefore, he joins a good conscience to confession. For we see that many are quick enough with their tongue, and talk idly a great deal, very freely, yet with no fruit, because their life does not correspond.

Moreover, the integrity of conscience alone is what gives us confidence in speaking as we should. For those who talk idly a great deal about the gospel, and whose immoral life is proof of their impiety, not only make themselves objects of ridicule but also expose the truth itself to the slanders of the ungodly.

For why did he previously instruct us to be ready to defend the faith, if anyone should ask us for a reason for it, if not because it is our duty to defend God's truth against the false suspicions that the ignorant hold concerning it? But the defense of the tongue will achieve little, unless one's life corresponds with it.

He therefore says, that they may be ashamed, who blame your good conduct in Christ and who speak against you as evildoers. It is as if he had said, “If your adversaries have nothing to allege against you, except that you follow Christ, they will eventually be ashamed of their malicious wickedness, or at least, your innocence will be sufficient to refute them.”