Well, I thank God for the cross of Christ! The cross allows us to be declared righteous without having to depend on the righteousness of the law, which as God says are only like filthy rags. With this in mind, I trust that whenever I sin or anybody else, no matter how many times or for how long I have been doing it, the cross of Jesus Christ is enough to satisfy the wrath of a holy God.
I think what is most important is that we desire holiness, not that we always achieve it, although we must never forget that with the new life we have it is possible to say "no" to sin, we must also remember that sin inevitably will entice and we will fail. It will happen because we are fallen and we have a evil nature that refuses to die although we are to declare it as so (dead).
No matter what though, no matter how many times we sin, we must always come back to the cross and know that whatever we have done it can and has been covered by the blood of Christ. This does not give us a license to sin, but on the contrary, in view of such awesome and loving grace how could we want to continue to sin?
So the point is: if we love God and have been saved by His Son we will not live in sin (this means consciously sinning and never repenting over it). But is also means that if we do sin we will always return to the cross and to mercy to receive His grace in our time of need, He knows we are fallen and the greater the sinner, greater is the grace that is bestowed on him/her as long as he/she believes.
I still struggle with sin, although I hate the sin I do and desire to stop, I still do it sometimes and I always regret it and repent over it. I desire holiness in my life but I can never achieve it, but I know it is possible through Jesus and I believe that this desire to live holy and pure is the Holy Spirit working in my life. Man cannot desire holiness without the workings of the Spirit of God, so if you have this desire in you, you can know that you have been born of God.
All of mankind has taken advantage of the grace of God, it would not be grace if it was deserved (the very meaning of grace is that it is nothing that we can lay claim to best on our effort), but the love of God is not dependent on our performance, it is dependent on Jesus Christ and what He freely offers humanity. Forgiveness is always possible through the cross of Christ, we must though be willing to accept it instead of wallowing in self pity, every time we fail, we must repent and try again, we will not go anywhere but backwards if we refuse to let our Father forgive us, He always wants to forgive those who trust in His Son for salvation, we are God's children now, no longer His enemy, He deals with us as such (as a godly Father would deal with a son or daughter when they mess up).
He demands perfection but perfection is only brought about through the blood of Jesus. As long as you recognize this, you are on safe ground.
But I only know a few chapters in Romans where Paul addresses this issue to the Romans/Roman churches, are there any other chapters in the Bible regarding this? 
I evidently need multiple scriptural evidence to convince a friend. Please help! 
