Martin Luther on Obtaining Grace

The person who believes that he can obtain grace by doing what is in him adds sin to sin so that he becomes doubly guilty.

On the basis of what has been said, the following is clear: While a person is doing what is in him, he sins and seeks himself in everything.  But if he should suppose that through sin he would become worthy of or prepared for grace, he would add haughty arrogance to his sin and not believe that sin is sin and evil is evil, which is an exceedingly great sin.  As Jer. 2[:13] says, "For my people have committed two evils: they have forsaken me, the fountain of living waters, and hewed out cisterns for themselves, broken cisterns, that can hold no water," that is, through sin they are far from me and yet they presume to do good by their own ability.

Now you ask, "What then shall we do?  Shall we go our way with indifference because we can do nothing but sin?"  I would reply, By no means.  But, having heard this, fall down and pray for grace and place your hope in Christ in whom is our salvation, life, and resurrection.  For this reason we are so instructed – for this reason the law makes us aware of sin so that, having recognized our sin, we may see, and receive grace.  Thus God "gives grace to the humble" [1 Pet. 5:5], and "whoever humbles himself will be exalted" [Matt. 23:12].  The law humbles, grace exalts.  The law effects fear and wrath, grace effects hope and mercy.  "Through the law comes knowledge of sin" [Rom. [3]:20], through knowledge of sin, however, comes humility, and through humility grace is acquired.  Thus an action which is alien to God's nature results in a deed belonging to his very nature: he makes a person a sinner so that he may make him righteous.

From the Treasury of Daily Prayer, Concordia Publishing House, 2008, p. 731.