We are branded as sinners because of Adam and Eve. Sure they were the first people that God made, but God gave them free will do whatever they want to do. Except for giving them the boundaries of not getting any fruit from the tree of the knowledge of good and evil. Think about it... If they would not have done that we could not know what wrong and right is.
If we were to say that we were not sinners than we would not believe that we still do bad things. Even though we do accept Jesus as our Lord and Savior of our sins (forgiving us of our sins) God still gives us the free will to either Truly live for Him or to live according to everything the world does. For instance living in the life of sin, or living Holy Lives away from sin living a life pleasing to God.
Just because He forgives our sins does not mean that we can do whatever we want. Remember, when Adam and Eve ate of the fruit of the tree of the Knowledge of Good ad Evil their eyes were open to know that they were physically naked in the sight of God (they were to start out with even before they ate of the fruit), but they just didn't know that because they didn't think there was anything wrong about it. That's why they hid themselves in the bushes and tried to hide their bodies with fig leaves.
This is the same thing God does for us... God is the same, he never changes, He is the same from that time to now, always and forever. He gives us grace to do whatever we want. But because He loves us he will still "judge us" just like a parent may punish a child for doing wrong. Because they do wrong and in the process may hurt themselves or others because of what they do. Since the parents love them, they punish them so that they will not hurt themselves or others anymore. Giving them discipline next time for what not to do in the future and giving them reasons of why they should not do that anymore.
God can and will tell us why we should not do certain things, because it may hurt others or yourself as well. God can give us Grace that way in a way that we can see for ourselves of his Truth and faithfulness. If we live in Him, living True godly lives God will give us Blessings that we may never of even thought of. He loves us and that is why we are branded as sinners so that we know that we can still do things that are wrong, humbling ourselves to know that can't do everything without us or others getting hurt in some way, but know that if we live the godly He REALLY wants us to live He will Bless us because of the way we are living.
It's not by works and works alone, it's in the love that we show and share to others. Remember...they will know us by are love and unity with one another.