I am always amused when someone uses that old 'the demons believe and tremble, but that doesn't save them' line. The demons have no faith because they see Jesus for Whom He is, as attested by the demons Jesus cast out into the swine herd, etc. We exhibit faith when we believe in Him, and it is faith erupting from the broken and contrite spirit that God acts upon to make us/them born from above, as Jesus taught Nicodemus.
They aren't saved and can't be saved because they're not human.
Jesus died for mankind. Salvation was never offered to the demons. THAT'S why they can't be saved even if they believe in Him, ie. know He exists and recognize who He is.