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Satan
In plain words
Satan is a real spiritual enemy of God and people, already defeated by Jesus.
The Bible says there is a real spiritual enemy called Satan, which means accuser, or the devil. He is not God's equal opposite. He is a created being who rebelled. He lies, tempts and accuses, trying to pull people away from God.
This is not a reason to be scared or to see demons everywhere. The Bible is calm about it. Jesus came to undo the devil's work (1 John 3:8), and on the cross He disarmed the powers of evil (Colossians 2:15). The enemy is real, but he is beaten.
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Peter says to stay alert, because the devil looks for someone to devour, and to resist him, standing firm in faith (1 Peter 5:8-9). James adds: "Submit yourselves, then, to God. Resist the devil, and he will flee from you" (James 4:7). Notice the order: first God, then resisting.
Two mistakes are common: ignoring Satan completely, or blaming him for everything. Most of the time the Bible points to our own desires as the main battle. Practically, Satan's main weapon is lies, so answer them with truth. When you hear "you are worthless" or "God is holding out on you", name the lie and answer with a Bible verse.
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The Hebrew satan means adversary or accuser, and the Greek diabolos means slanderer. In the Old Testament he appears in a few places as an accuser. In the New Testament his defeat by Christ is clear, and his final end is certain (Revelation 20:10). He has no future.
Christians differ on some details, such as how demonic influence works today and how to deal with it; follow wise leaders in your local church and do not seek out the occult. All agree that Satan is a defeated enemy, that believers belong to Christ, and that God remains completely in charge.
My verses
Be sober-minded and alert. Your adversary the devil prowls around like a roaring lion, seeking someone to devour. Resist him, standing firm in your faith and in the knowledge that your brothers throughout the world are undergoing the same kinds of suffering.
Submit yourselves, then, to God. Resist the devil, and he will flee from you.
And having disarmed the powers and authorities, He made a public spectacle of them, triumphing over them by the cross.
The one who practices sin is of the devil, because the devil has been sinning from the very start. This is why the Son of God was revealed, to destroy the works of the devil.
And the devil who had deceived them was thrown into the lake of fire and sulfur, into which the beast and the false prophet had already been thrown. There they will be tormented day and night forever and ever.