It's more than just believing He exists (God and Jesus being the same individual briefly splitting into two seperate, but connected through the Holy Spirit, beings during Jesus' time on Earth), it's believing what He stands for and living accordingly.
Satan is not saved technically or otherwise, Satan is DOOMED! After the thousand years of imprisonment, Satan will be released for a short while. During his parole, he will again unite some of the nations and take them to war against God's people (sounds a bit like Osama bin Laden doesn't it?!). But this rebellion will be summarily ended when God sends fire out of heaven to destroy them. Because he is spirit and cannot die, the Devil will then be sentenced to eternal torment in the Lake of Fire. Finally, God and man will be rid of their chief enemy, and peace will flourish for all eternity! - Revelation 20:7-10.