The Grace Of God/The Free Gift

"If you are a preacher of Grace, then preach a true, not a fictitious grace; if grace is true, you must bear a true and not a fictitious sin. God does not save people who are only fictitious sinners. Be a sinner and sin boldly, but believe and rejoice in Christ even more boldly. For he is victorious over sin, death, and the world. As long as we are here we have to sin. This life in not the dwelling place of righteousness but, as Peter says, we look for a new heavens and a new earth in which righteousness dwells. . . . Pray boldly-you too are a mighty sinner.
 If you are a preacher of mercy, do not preach an imaginary but the true mercy. If the mercy is true, you must therefore bear the true, not an imaginary sin. God does not save those who are only imaginary sinners. Be a sinner, and let your sins be strong, but let your trust in Christ be stronger, and rejoice in Christ who is the victor over sin, death, and the world. We will commit sins while we are here, for this life is not a place where justice resides. We, however, says Peter (2. Peter 3:13) are looking forward to a new heaven and a new earth where justice will reign. It suffices that through God's glory we have recognized the Lamb who takes away the sin of the world. No sin can separate us from Him, even if we were to kill or commit adultery thousands of times each day. Do you think such an exalted Lamb paid merely a small price with a meager sacrifice for our sins? Pray hard for you are quite a sinner.”

Martin Luther    From his letter to Melancthon August 1521

Romans 3:5  But if our unrighteousness serves to show the righteousness of God, what shall we say? That God is unrighteous to inflict wrath on us? (I speak in a human way.) 6  By no means! For then how could God judge the world? 7  But if through my lie God's truth abounds to his glory, why am I still being condemned as a sinner? 8  And why not do evil that good may come?--as some people slanderously charge us with saying. Their condemnation is just. 

Luther is not advocating licentiousness, of which Paul was also accused. He simply states the case, (as did Paul), that God's grace freely given through His Son, to the elect, is sufficient.
What sin did Christ not die for? Some say He did not die for "the sin of unbelief" ??!!
The truth is that if one never comes to belief, none of his sins are forgiven. Christ died solely for the sins of the elect, and the assurance is the elect will all come to belief. Thus the sin of unbelief is a moot point. (John 6:35-47; 17:1-3)

Christ's death is sufficient to forgive every sin of the elect,not just the sins of our past, but the sins we fall into today, and in the future. Luther is not telling us to go and commit a thousand sins. He is encouraging us to rely solely on Christ and His finished work, i.e. His obedient act, and not on any works of our own. Even if we were to commit a thousand wrongs, God's grace is eternal, sufficient and effectual, never reaching a "limit" where we can lose that which the Son has wrought for us, to us, and in us.

Those who teach the sinless perfection of the saints, as the standard of "keeping" one's salvation preach heresy, since Christ's death as propitiation is the only act of a man that the Father will accept.
He did not die so that any who "make a decision for Him" can have their slate wiped clean and start all over again in an endless cycle of breaking the law in an attempt to keep it.(Romans 7)

Galatians 5:4  You are severed from Christ, you who would be justified by the law; you have fallen away from grace. 

Since Christ died for the forgiveness of every sin of the elect, then it is not our sins that separate us from the Father, (else His death is meaningless), but it is our efforts to justify/sanctify ourselves that cause us to fall from grace. This is the "original sin"; that man would be His own god and thus his own savior. (Genesis 3:1-8; v22)
So,sin boldly as you are a sinner,but pray more boldly rely on Christ more boldly as you are saved by grace.

Solus Christus
Sola Gratia
Soli Deo Gloria