That's why it's so important to preach the Gospel, even to the saved, to keep it fresh in our minds so we can be an effective witness for Christ when someone gives us the opportunity. Keep a clear conscience, so that those who speak maliciously against your good behavior in Christ may be ashamed of their slander. The believer must have a clear conscience. If we know we are innocent of wrongdoing, we can go through persecution with confidence. If we have a bad conscience, we'll have feelings of guilt and won't be able to witness for the Lord.
Conscience is the ability to evaluate the right or wrong of something and to act accordingly.
Our opposition may try to make the Christian look bad and slander our good conduct, but a good conscience won't let us be adversely affected by that. Our good conduct in Christ is acting as He would act, even acting in union with Him. Our conduct should speak for itself and when it does it will put them to shame. Keeping a good conscience may not alter the situation of the suffering Christian. Suffering for righteousness can't always to be avoided.