The line is perhaps a little funny, and usually meant to be so. But I wonder. Can we discount the reality of the adversary? The the answer can be found in this jewel of a quote from Nephi (2 Ne 28:22). One of the Devil’s best moves has been to convince people that the horned, cloven-footed guy with a funny tail doesn’t exist and there are no consequences to breaking the commandments. The really clever move was to etch the caricature of him so deep in our culture – and thus our collective subconscious – even while his existence was not in doubt that it’s hard to remember sometimes that he is in fact nothing like the popular culture has portrayed for the last millennium or so. I don’t know what he looks like, but I suspect he wouldn’t raise too many eyebrows if we were to see him around here. It’s been said that he can appear as an angel of light (2 Ne 9:9), so I don’t expect him to advertise his intentions by his appearance.
Just as there is a loving God, who wants his children to be happy, there is the antagonist, who wants us to be miserable like he is. It is necessary that there are both forces in the world, or we wouldn’t be able to act for ourselves.
We must remember that our adversary only has as much power over us as we allow him. Just as God won’t force us to act in a certain way, the Devil can’t. Remember, we are free to act for ourselves. There are some circumstances where people lose control of their faculties to a degree or for some time. That can happen through willful surrendering of our willpower to the adversary (like intoxication or persisting addictive habits) or perhaps some types of mental illness. I have experience with depression, and it seemed to me that it did limit my ability to control myself (medication was a different kind of problem, but still a problem).
The Scriptures and the living prophets tell us about how the Devil works, and we should heed the warnings. Other than that, we are much better off learning how we can develop Christlike attributes in our lives than by obsessing about the adversary. Again, it is in his interest to say there is no Devil. It is also in his interest to get us so fascinated by him that we exclude worthy pursuits.
Jesus said that you can not serve God and mammon. The living prophets have talked about how we have to choose our side. Some of us want to be neutral, but that is impossible.
Yet it is true that in many instances our choices are not between good and evil. They can be between good and best, but that is another story. We can use our own reason and seek guidance from the Spirit.