Little children, let no one deceive you. Whoever practices righteousness is righteous, as he is righteous. Whoever makes a practice of sinning is of the devil, for the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the works of the devil.(1 John 3:7–8)

When 1 John 3:8 says, “The reason the Son of God appeared was to destroy the works of the devil,” what are “the works of the devil” that he has in mind? The answer is clear from the context.

First, 1 John 3:5 is a clear parallel: “You know that he appeared in order to take away sins.” The phrase he appeared to occurs in verse 5 and verse 8. So most likely the “works of the devil” that Jesus came to destroy are sins. The first part of verse 8 makes this virtually certain: “Whoever makes a practice of sinning is of the devil, for the devil has been sinning from the beginning.”
The issue in this context is sinning, not sickness or broken cars or messed up schedules. Jesus came into the world to enable us to stop sinning.

We see this even more clearly if we put this truth alongside the truth of 1 John 2:1: “My little children, I am writing these things to you so that you may not sin.” This is one of the great purposes of Christmas — one of the great purposes of the incarnation (1 John 3:8).
But there is another purpose which John adds in 1 John 2:1–2, “But if anyone does sin, we have an advocate with the Father, Jesus Christ the righteous. He is the propitiation for our sins, and not for ours only but also for the sins of the whole world.”
But now look what this means: It means that Jesus appeared in the world for two reasons. He came that we might not go on sinning — that is, he came to destroy the works of the devil (1 John 3:8); and he came so that there would be a propitiation for our sins, if we do sin. He came to be a substitutionary sacrifice that takes away the wrath of God for our sins.

The upshot of this second purpose is not to defeat the first purpose. Forgiveness is not for the purpose of permitting sin. The aim of the death of Christ for our sins is not that we relax our battle against sin. The upshot of these two purposes of Christmas, rather, is that the payment once made for all our sins is the freedom and power that enables us to fight sin not as legalists, earning our salvation, and not as fearful of losing our salvation, but as victors who throw ourselves into the battle against sin with confidence and joy, even if it costs us our lives.

– John Piper