What being a priesthood of believers means is that God's saving grace not only is the answer or remedy for our sin it has as its purpose that in being restored to God we are now to serve him. We are not our own, we have been bought with a price and due to his grace we are to desire to render service to Him for his glory and praise. We serve because we have been justified, we don't serve to secure justification. So our service to Him entails offering spiritual sacrifices.