The “devil” (“Satan”/“Lucifer”), “demons” and “angels” (“fallen” or not) appear to exist no more than does “God”, which is not at all. The entire cosmology appears to be nonsensically held.
Not according to the Roman emperor Constantine, who struck a coin in 313 A.D. in recognition of his god "Sol Invictus", for help in winning the battle of 312 A.D. for him. It is that same god, "dragon"/"devil", who Constantine recognized in the year 321 A.D. by changing the law for the day of rest to be on the day of the sun, Sunday, in recognition of his god, Sol Invictus, whose birth date is the 25th of December. It is the same church he set the dogma for in 325 A.D., the Roman Catholic church, which his kingdom supported, which still exist today, and a daughter of that church, in England, just crowned their new king last week. You may consider kings, and tyrants nonsensical, but as with Putin, and Xi, they surely exist, and they worship the gods of war and armaments, and they affect great masses of people. The U.S. has their own tyrant, in the form of an elderly, corrupt, incompetent, by the name of Biden, apparently with his own demons, as is the case with Pelosi, who had to hire someone to cast demons from her own house. You can say demons don't exist, but then again, you can say that craziness does not exist. In both instances you would be wrong. You can say alpha, beta and gamma rays don't exist because you can't see them directly, but they can kill you none the less.