Evidence for the existence of Satan
"Why do I believe in the devil? For three reasons. 1. Because the Bible plainly says he exists. 2. Because I see his work everywhere. 3. Because great scholars have recognized his existence."
Atheists are helped by Satan
A rather imaginative argument is based on atheists and their secular agenda: 
- Proselytizing requires external encouragement
- Atheists proselytize
- God is not encouraging atheists to proselytize
- The only explanation is Satan is encouraging atheists to proselytize
- Satan exists
Great scholars believed in Satan
Other religions believe in a powerful demonic force
Possession and personal experience
- "The devil does not have a body, a color, or a visual form, nor does he have horns, wings or a tail. He is an invisible, bodiless entity. [...] I would be lying if I did not acknowledge that I felt that evil presence in an intense and powerful way. [...] Although we tend to speak of the demon, in reality there are many demons, each one is different, but there is one who is head over all the demons, the most powerful one: Satan "