Yes, Jesus has defeated the enemy, that's for sure. But the devil is still trying to exercise power in this world, you have to be blind not to see that all around us every day. There is murder, abuse, violence, and misery... everywhere.
He has power because people give it to him. Satan is always looking for people through whom to do his destructive work. That doesn't sound very encouraging, of course.
And frankly, it would be very depressing if we didn't understand (or know) that God is also always looking for people through whom He can do His redemptive work! We need to understand those two things. In 1 John 5:19 it says: We know that we are children of God and that the world around us is under the control of the evil one.
This was written some 2,000 years ago! The whole world is under the power of the devil. That may sound very frightening, but it is exactly why it is so important to know whether you are a child of God. We are all part of God's creation, but not automatically His child.
In John 1:12 it says that we have been given the right to become children of God. πHow? By accepting Jesus and believing in His name, it says. That's not very complicated. With that in mind, it is very good to continue our read in 1 John 5:20,
And we know that the Son of God has come, and he has given us understanding so that we can know the true God. And now we live in fellowship with the true God because we live in fellowship with his Son, Jesus Christ. He is the only true God, and he is eternal life.
From such a perspective, we can look at this world full of hope!