Grace and gratitude belong together like heaven and earth. Grace evokes gratitude like the voice an echo. Gratitude follows grace like thunder lightning. Not by virtue of any necessity of the concepts as such. But we are speaking of the grace of the God who is God for man, and of the gratitude of man as his response to this grace...
Radically and basically, all sin is simply ingratitude — man’s refusal of the one but necessary thing which is proper to and is required of him with whom God has graciously entered into covenant. As far as man is concerned there can be no question of anything but gratitude; but gratitude is the complement which man must necessarily fulfill.
- Karl Barth