“I know there are some in the world that deny the existence of a personal Satan. This denial is false, being sponsored by the father of lies himself. The fact is, however, that Lucifer is a personage of spirit, just the same as Jesus and you and I were personages of spirit before we were born. Now, we may rest assured of this: if there is no devil there is no God.”

—	Marion G. Romney, Ensign October 1983, page 4

The biggest advantage the adversary has on us in these days is that we're gullible. What better practice range for his devoted literal fallen angels than a bunch of people that are determined to believing that he and they don't exist at all?
It takes very little to cause people that don't know any better to sin at all, and that's yet another reason that those that know the truth and know how to resist and overcome temptation have to continually resist it and set an example.
I can bear personal witness that giving in to sin and succumbing to it is never a good idea.