I frankly confess that, for myself, even if it could be, I should not want “free-will” to be given me, nor anything to be left in my own hands to enable me to endeavour after salvation; not merely because in face of so many dangers, and adversities and assaults of devils, I could not stand my ground; but because even were there no dangers, I should still be forced to labour with no guarantee of success.
But now that God has taken my salvation out of the control of my own will, and put it under the control of His, and promised to save me, not according to my working or running, but according to His own grace and mercy, I have the comfortable certainty that He is faithful and will not lie to me, and that He is also great and powerful, so that no devils or opposition can break Him or pluck me from Him.
Furthermore, I have the comfortable certainty that I please God, not by reason of the merit of my works, but by reason of His merciful favour promised to me; so that, if I work too little, or badly, He does not impute it to me, but with fatherly compassion pardons me and makes me better. This is the glorying of all the saints in their God.
Source: Martin Luther, The Bondage of the Will (eBook)