"...keeping a clear conscience, so that those who speak maliciously against your good behavior in Christ may be ashamed of their slander" (NIV)
Written by the apostle Peter, this verse is the essence of Christlike behavior. Peter wrote this letter to offer encouragement and advice to fellow-believers who had been driven out of Jerusalem and scattered throughout Asia Minor. This was when Nero was Emperor of Rome and there was great persecution of Christians. (In fact, Peter was eventually executed during this time.)
In this scripture Peter is saying that our behavior should demonstrate the change in us that Christ has made. Our faces should reflect His love. We can quietly demonstrate our faith by being polite in a bad situation, by leaving a good tip for the waitress, by patiently waiting in the checkout line... little things that can make a big statement.
In the previous verse, I Peter 3:15, Peter advises: "But in your heart set apart Christ as Lord, always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have. But do this with gentleness and respect..." We can always be ready to answer questions about our faith. Be prepared to tell people what God has done in your life. You don't have to be a preacher, or memorize oodles of scriptures, or be able to sing, or do anything special; all you have to do is love the Savior and be willing to talk about that. The words will come.
But, do remember that the people asking the questions may not always like your answers! Or, they may need time to ponder what you have said. Peter's advice is to answer with gentleness and respect. There may still be those who will be spiteful or hateful. In those cases you can only maintain your standards as a Believer... and, trust that these people's criticisms and slanderous talk will not be believed. This is definitely not easy, but when you are going through this, you are not alone...
"Jesus answered, '...Yet I am not alone, for my Father is with me. I have told you these things, so that in me you may have peace. In this world you will have trouble. But take heart! I have overcome the world.' "