By definition, the big difference between mercy and justice is that mercy is never ever obligatory.
R. C. SproulFor the soul of a person to be inflamed with passion for the living God, that person's mind must first be informed about the character and will of God. There can be nothing in the heart that is not first in the mind. Though it is possible to have theology on the head without its piercing the soul, it cannot pierce the soul without first being grasped by the mind.
R. C. Sproul