“Be sober, be watchful. Your adversary the devil prowls around like a roaring lion, seeking some one to devour.” (1 Peter 5:8)
It's easy in these enlightened times not to think much about the devil, or, if we do think about him, to think that he doesn't exist. We ignore or deny the existence of the devil, however, at our peril. To quote Baudelaire's oft-paraphrased line, "La plus belle des ruses du diable est de vous persuader qu'il n'existe pas." (The finest trick of the devil is to persuade you that he does not exist.) The Bible mentions Satan by name nearly 50 times, not to mention all the times he is referred to by some other word or phrase. Indeed, the devil is all too real, and it is only when we acknowledge this that we can fight him and his evil influence in our lives and in the world. As St. Paul writes:
"Finally, be strong in the Lord and in the strength of his might. Put on the whole armor of God, that you may be able to stand against the wiles of the devil. For we are not contending against flesh and blood, but against the principalities, against the powers, against the world rulers of this present darkness, against the spiritual hosts of wickedness in the heavenly places. Therefore take the whole armor of God, that you may be able to withstand in the evil day, and having done all, to stand. Stand therefore, having girded your loins with truth, and having put on the breastplate of righteousness, and having shod your feet with the equipment of the gospel of peace; besides all these, taking the shield of faith, with which you can quench all the flaming darts of the evil one. And take the helmet of salvation, and the sword of the Spirit, which is the word of God." (Ephesians 6:10-17)
(Side note: When I typed this out, I accidentally wrote about the "flaming farts of the evil one." Now that would be really scary.)
The devil wants to lead us to destruction, to keep us from experiencing the amazing love of God, and to that end he works in all kinds of terrible and subtle ways: by persuading us that God doesn't exist (or at least doesn't matter), or that prayer is boring or pointless, or that sin isn't really sin, or any number of other things. And he is quite adept at disguising his attacks so that we never suspect he's behind them -- he can make the most heinous lies seem utterly reasonable, and he can lead us astray so gradually that we're a million miles from home before we realize we've left at all. That is why we must be always on our guard against the devil.
And, luckily, we're not alone in the battle. When we are tempted by the devil, God will always give us the grace to resist him if we are willing to accept it. God even has a whole army of angels ready to do battle with the devil, headed by St. Michael the Archangel, to whom we can also pray for backup:
"St. Michael the Archangel, defend us in battle; be our protection against the wickedness and snares of the devil; may God rebuke him, we humbly pray; and do thou, O Prince of the heavenly host, by the power of God, thrust into hell Satan and all evil spirits who wander through the world for the ruin of souls. Amen."