In today’s vocabulary the term “conscience” refers to the inner court of appeal that renders judgment on our own behaviour (1 Peter 3:16). Peter, however, uses the term to describe a right attitude and/or behaviour before God. In the context of baptism this “appeal to a good conscience” amounts to a request to God to give strength to live in a “washed” fashion, that is, consistent with the forgiveness of sins. Recall that Peter in this paragraph has been drawing out the necessity of doing good even if it leads to suffering. The temptation to do less than good to spare yourself pain is enormous. By reminding his readers of their baptism (and hence its purpose = “appeal”) Peter motivates them to maintain a good conscience, that is, persevere in good deeds.
21 Baptism, which corresponds to this, now saves you, not as a removal of dirt from the body but as an appeal to God for a good conscience, through the resurrection of Jesus Christ,