Some spiritual secularists and others believe in evil spirits but not in Satan.
Some may need evidence of this in scripture.
The Bible distinguishes Satan from other demons for us.
How does the Bible do this?
In the Gospels, Jesus heals those afflicted by demons; in some cases the demons leave the individual and enter some other Corpus. At the Last Supper, Satan enters Judas Isacariot. Clearly, the Bible distinguishes Satan from demons.