Your point re: The Screwtape Letters is well made; however, it also bears repeating that Satan is not now, nor has he ever been omnipresent, omniptent, or omniscient. Using the logic that evil is everywhere is an alarmist and extreme view which is incorrect. Evil IS present in the world, but as faithful Christians, we need not fear it. Not fearing a thing and not believing a thing exists are radically different notions. I don't fear Satan, but I believe he and his demons exist. Don't fall prey to those who would have you believe that evil must be feared. That's what Satan wants. Luther once said (at least I think it was him) that the one thing that Satan cannot stand is derision and laughter. The devil desperately wants to be taken seriously, and if we fear everything because of its potential for evil use, then it seems to me that he is winning. But, on the other hand, if we ridicule Satan because of his powerlessness over us, he loses. All of this must be done by God's through us, lest I be accused of preaching synergism.
"Satan is not now, nor has he ever been omnipresent, omniptent, or omniscient. Using the logic that evil is everywhere is an alarmist and extreme view which is incorrect."
True, but Satan is smarter and more powerful than any human, and he has lots of help. Remember, he and his entire army of rebel angels were cast out of heaven, and wander the world seeking the ruin of men's souls.
Insofar as they lead people astray, those people become part of the "auxillary" and a force for evil in their own right.
With all of that going on, we are today pretty much surrounded by evil, and tempted daily.