Don't kid yourself...
Secular Humanists don't believe in the supernatural by definition. Quoting the the Council for Secular Humanism:
Secular humanism is philosophically naturalistic. It holds that nature (the world of everyday physical experience) is all there is, and that reliable knowledge is best obtained when we query nature using the scientific method. Naturalism asserts that supernatural entities like God do not exist, and warns us that knowledge gained without appeal to the natural world and without impartial review by multiple observers is unreliable.
This would certainly rule out belief in Satan, wouldn't you agree?