Because we live in a world that is under the influence of the devil and the kingdoms of this world — we face a world system that is opposed to and battles against everything that is of God and that is according to His Word. However, as a Christian, you believe that Jesus is the Son of God and you therefore have what it takes to overcome this world.
If Jesus has already conquered every enemy, why do you need to do anything to overcome the world? Or, more to the point, what could you even do to overcome the world? There is no doubt God created you to win and to be an overcomer and more than a conqueror, but in your own strength and abilities that would not be possible.
To overcome this world, you must be born of God: you must be born again as Jesus put it. God created you to have dominion over the devil but because you are not perfect, the devil has a claim that can only be satisfied by someone who is willing to pay the price for your sin; your imperfections (not just yours but everyone’s for all have sinned).
If you receive the gift of salvation but continue to live and to think and to talk as though you have not been born again, then you are giving the enemy a victory that Jesus has already rightfully won. Jesus always wants to lead you in a victory procession, triumphing over the enemy, but if you are still listening to the devil’s lies about your past you are not walking in victory. You cannot go forth in victory with the Lord while holding on to the past and things can be overwhelming if you compare your abilities to what is required for the battle.
The only victory that has overcome the world is your faith in Jesus Christ and His finished work for you. He has already won the victory for you over the devil and over sin; you can’t win it for yourself or somehow pay Him for the victory — you have to receive it by faith. When you believe that Jesus paid for your past and made you an entirely new person with all of the old gone and everything new, then you overcome the world.
God created you to reflect His glory and to testify of His faithfulness. From the beginning of time His purpose in the creation of mankind was, “very good”. You were created to win!