You may not realize how much of our modern conception of a "devil" is derived from pagan symbols. Horns are symbols of fertility, the trident was the tool of Poseidon. Sometimes Satan is depicted as a faun, and fauns were associated with mischief and orgies. Even the pentagram started out life as a Wiccan symbol of faith. But the church deemed these symbols as threats and "demonized" them in perhaps the most literal sense of the word. The most Satan-like compilation of these components is Baphomet, who was invented by Christians--not as a Christian devil but as a pagan or Islamic deity and then became a devil.
Even today the church makes potential threats into demons, often as not targeting symbols of fertility as being the most threatening. This makes sense, of course: sex is a powerful narcotic. Why do you think the church is so hell-bent (ahem) on controlling and rationing the sexuality of its patrons? Why do you think we have such widespread fear of our own genitalia? Hell, I've even heard Christians describe peace symbols as "Satanic" because they are upside-down broken crosses, when in reality the threat comes from the association of the 60's peace movement with "free love".
This is all well and good, and fairly forward-thinking, but the church needs to step up its game and look to modern threats. So if there's anyone out there that has influence over Christian iconography, I submit the following symbols. They represent threats to Christianity and need to be rebranded as demonic.
Understanding medicine takes away the need to pray for healing, so this one has got to go. See, it's already got the snakes, so we're halfway there. And with the wings, I can see "serpent angel" as a symbol of Satan working pretty well.
Ah, Science. Who needs religion to tell you how the world works when you can have actual facts? So, how to make this one evil... maybe by relating it to the pentagram... or by thinking of electron orbits as circles of Hell.
Genetics, just as potent. How can you deny evolution with all this evidence of it laying about? Better demonize this symbol as well, and thankfully it's easier--call it a twisted ladder to the underworld. Am I right?
This one is a bit trickier, but ultimately necessary. The Erlenmeyer Flask is perhaps the most prescient representation of experimentation. It symbolizes the epistemology that separates science from religion, for science is based on gathering knowledge and perfecting it through revision and observation--whereas religion is the resistance of new knowledge and learning in lieu of the unwavering fortification of superstitions and myth. So the flask has got to go. Satan-ifying this one is not nearly as straight-forward, but I think the downward-taper of the flask could be a statement that Earth is larger than Heaven. Will have to give this one more thought... erm... prayer, I mean.
Just pure evil, obviously. Maybe you could argue that the brain stem and cerebellum sort of look like a penis and scrotum if you turn your head just right, but that doesn't quite work for me. On the other hand, maybe it doesn't need to be a symbol for anything. Maybe it's enough that the brain not only represents but physically manifests the gathering of knowledge, the housing of the mind, thought, will, desire, action, and--therefore--sin. Maybe its enough that spiritual events--feeling god's presence, near-death glimpses of heaven--can be shown as reproducible products of the brain. Maybe it's enough the brain is by nature antithetical to faith, making it unholy by definition.
Or we can always make up something scary later.