Having a good conscience; that, whereas they speak evil of you, as of evildoers, they may be ashamed that falsely accuse your good conversation in Christ. Having a good conscience - The testimony of God in your own soul, that in simplicity and godly sincerity you have your conversation in the world. See on the term conscience at the end of Hebrews.
Whereas they speak evil of you - See the same sentiment in 1 Peter 2:11 and the note there.