Speaking entirely from a faith based perspective:
I believe on Judgement Day you will only have two choices: to serve Satan or to serve the Lord of Hosts through Jesus Christ. Satan may be a believer, but that does not necessarily make him saved. To be saved you have to follow the teachings of Jesus Christ; it is called applied faith. Everyone has faith, whether it is in Christianity, Judaism, science, love, friendship, their country, the human race itself, etcetera- but without application, faith means nothing. Satan's desire, from what I presently understand, is to undermine and acquisition the authority of the Lord and become the king over mankind and to establish his throne in heaven. To do so, he would have to be a cunning, wicked and clever individual. I wont get into the details of how he will attempt to win over the minds of those without faith in the Lord nor those who claim to be in service to the Lord; but if you place your faith in the Lord of Hosts he will give you the signs you need and he will guide you onto the path of salvation.
Lastly, the Lord knows the minds of all men, women, children, animals, etcetera, therefore there are is not a soul in all of creation that is truly lost. Even those who have been cast down into the bottomless pit will one day return to the doorsteps of the Lord. Perhaps when most people use the phrase "lost soul" they are speaking entirely from a figurative point of view. If you apply your faith in the Lord of Hosts, you will be granted salvation through the Holy Spirit- that divine spark that resides within each of us who affirm to our faith. Please note that we do not need to wear our faith on our sleeves nor do we need to crusade in the name of a king.