Although God is not imputing our sins unto us, we cannot afford the luxury of sin because it allows Satan to have access to us. When a Christian sins and allows the devil opportunity to produce his death in their life, the way to stop it is to confess the sin, and God who is faithful and just, will take the forgiveness that is already present in our born again spirit and release it in our flesh. This removes Satan and his strongholds.
The sins of a Christian don't make him a sinner any more than the righteous acts of a sinner make him righteous. Sin is a very deadly thing that even Christians should avoid at all costs, but it does not determine our standing with the Lord. A person who is born again is not "in" the flesh even though he may walk "after" the flesh.
What is the motive for living a separated life? We live a separated life because our nature has been changed. We were darkness, now we are light (Ephesians 5:8). Many people argue for holiness in order to obtain relationship with God. We need to live holy lives because of the relationship that we already have. It's the nature of a Christian to walk in the light and not in darkness. If Christians were rightly informed of who they are and what they have in Christ, holiness would just naturally flow out of them. It's their nature. It's our nature.