When God makes us into a new creation, he gives us a new purpose of being his witnesses and his ambassadors to the world. So how do we do that on a practical and tangible basis? Paul instructed the church of Philippi to "work hard to show the evidence of your salvation" (Phil 2:12). This means that we have a part in changing our behaviours and our lives to reveal Jesus to the world.
Changing our habits and our behaviours is hard, but not impossible.