This is my favourite passage outside of the bible. I quote it so often I need to put it here so I can find it quickly.
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 and hold fast my ‘free-will’ (for one devil is stronger than all men, and on these terms no man could be saved); but because, even were there no dangers, adversities, or devils, I Should still be forced to labour with no guarantee of success, and to beat my fists at the air. If I lived and worked to all eternity, my conscience would never reach comfortable certainty as to how much it must do to satisfy God. Whatever work I had done, there would still be a nagging doubt as to whether it pleased God, or whether he required something more. The experience proves that; and I learned it well enough myself over a period of many years, to my own great hurt. 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. ‘No one,” He says, ‘Shall pluck them out of my hand, because my Father which gave them me is greater than all’ (John 10:28-29). Thus it is that, if not all, yet some, indeed many, are saved; whereas, by the power of ‘free-will’ none at all could be saved, but every one of us would perish.
Furthermore, I have the comfortable certainty that I please God, not by reason of 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.
Martin Luther – The Bondage of the Will