Resist him, firm in your faith. (1 Peter 5:9)
The two great enemies of our souls are sin and Satan. And sin is the worst enemy, because the only way that Satan can destroy us is by getting us to sin.
God may give him leash enough to rough us up, the way he did Job, or even to kill us, the way he did the saints in Smyrna (Revelation 2:10); but Satan cannot condemn us or rob us of eternal life. The only way he can do us ultimate harm is by influencing us to sin. Which is exactly what he aims to do.
So Satan’s main business is to advocate, promote, assist, titillate and confirm our bent to sinning.
We see this in Ephesians 2:1–2: “You were dead in your trespasses and sins, in which you formerly walked . . . according to the prince of the power of the air” (NASB). Sinning “accords” with Satan’s power in the world. When he brings about moral evil, it is through sin. When we sin, we move in his sphere, and come into accord with him. When we sin, we “give place to the devil” (Ephesians 4:27).
The only thing that will condemn us at the judgment day is unforgiven sin — not sickness or afflictions or persecutions or intimidations or apparitions or nightmares. Satan knows this. Therefore his great focus is not primarily on how to scare Christians with weird phenomena (though there’s plenty of that), but on how to corrupt Christians with worthless fads and evil thoughts.
Satan wants to catch us at a time when our faith is not firm, when it is vulnerable. It makes sense that the very thing Satan wants to destroy would also be the means of our resisting his efforts. That’s why Peter says, “Resist him, firm in your faith.” It is also why Paul says that the “shield of faith” can “extinguish all the flaming darts of the evil one” (Ephesians 6:16).
The way to thwart the devil is to strengthen the very thing he is trying most to destroy — your faith.