Many people imagine a cosmic dualism: God and Satan as two roughly equal powers locked in eternal battle. That is not Christianity. The devil is a creature — finite, defeated, and already on borrowed time.
Where did the devil come from?
The Bible's portrait, drawn from Isaiah 14, Ezekiel 28 (read typologically by the Fathers), Luke 10:18, and Revelation 12: a created angelic being, made good, who rebelled against God through pride. He is not God's opposite; he is God's rebellious servant.
St. Augustine put it bluntly: "The devil, even unwillingly, is God's devil." Satan has no power except what God permits, and what God permits he limits and outwits (see Job 1–2).
Why allow him?
The same reason God allows any rebellious will to exist temporarily: love does not coerce. A universe in which "no" was instantly annihilated would be a universe in which "yes" meant nothing. God's solution to evil was not to vaporize evildoers but to undo evil from the inside — by becoming a man and absorbing its worst.
And there is a deeper answer: God uses the devil's malice for ends the devil never intended. Joseph's brothers meant evil; God meant it for good (Gen. 50:20). The devil drove Jesus to the cross; the cross destroyed the devil's power (Heb. 2:14). Satan is on a leash he keeps tightening on himself.
The decisive defeat
Colossians 2:15: "Having disarmed the powers and authorities, [Christ] made a public spectacle of them, triumphing over them by the cross." The Resurrection is the Day of Decision. The end is certain (Rev. 20). What remains is mop-up, in which we have been enlisted.
The Orthodox lens
The Fathers — especially the desert fathers like St. Anthony — urge us neither to dismiss demons nor to obsess over them. The weapons are simple: the Jesus Prayer, the sign of the Cross, the Eucharist, humility, confession, and love. Pride opens doors to demonic influence; humility closes them.
St. John Chrysostom: "The devil is chained as a dog. He can bark and growl, but he can only bite those who come within his reach." Stay close to Christ, and the leash isn't long enough to reach you.
A pastoral word
You do not need to live in fear of the devil. The defeat is signed in blood. Your job is to stay close to Christ — in prayer, sacraments, Scripture, community. The rest is already won.
And one more thing: most of what the devil actually does is much more boring than horror movies suggest. He whispers shame, despair, division, and lies. He works through Twitter and through resentment more often than through dramatic possession. Resist him in those quiet moments and he flees (James 4:7).