What was the word "world" meant in the eyes of John the Apostle?
In 1 John 2:15-16, he wrote;
15 Do not love the world or anything in the world. If anyone loves the world, love for the Father is not in them.
16 For everything in the world—the lust of the flesh, the lust of the eyes, and the pride of life—comes not from the Father but from the world. (NIV)
Was that all John had to the "world"? Not really, for in his gospel, he heard Jesus prayed, and he had this written in John 17:14-16
14 I have given them your word and the world has hated them, for they are not of the world any more than I am of the world.
15 My prayer is not that you take them out of the world but that you protect them from the evil one.
16 They are not of the world, even as I am not of it. (NIV)
John learned from Jesus' prayer, the world has the evil one, who want to keep them in the world. John put the same in his letter, 1 John 4:12-14 read;
12 I am writing to you, dear children,
because your sins have been forgiven on account of his name.
13 I am writing to you, fathers,
because you know him who is from the beginning.
I am writing to you, young men,
because you have overcome the evil one.
14 I write to you, dear children,
because you know the Father.
I write to you, fathers,
because you know him who is from the beginning.
I write to you, young men,
because you are strong,
and the word of God lives in you,
and you have overcome the evil one. (NIV)
Therefore in 1 John 5:4, "overcome the world" actually mean "overcome the evil". Then why not "overcome the sin"?
In Romans 5:12, Paul wrote sin entered the world when Adam ate the fruit from the tree of the knowledge of good and evil (Gen 3). So evil existed before sin, and sin was the consequence of the evil.
12 Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all people, because all sinned— (NIV)
We should take awareness, Jesus died on the cross for the forgiveness of our sin, not the forgiveness of our evil. And this was John written in 1 John 2:1
1 My dear children, I write this to you so that you will not sin. But if anybody does sin, we have an advocate with the Father—Jesus Christ, the Righteous One. (NIV)
He further wrote in 1 John 3:9
9 No one who is born of God will continue to sin, because God’s seed remains in them; they cannot go on sinning, because they have been born of God. (NIV)
It emphasized "No one who is born of God will continue to sin". If one continued to sin, then he is not born of God, but the evil one. Jesus overcome the evil one as the firstborn, in Him, we can do the same, that is "overcome the evil".
29 For those God foreknew he also predestined to be conformed to the image of his Son, that he might be the firstborn among many brothers and sisters. (Romans 8:29 NIV)