These two ideas are not normally thought of as going together very well. Justice demands that the full measure of punishment of the law be meted out in punishment for a crime committed. But justice is not simply limited to satisfying the law. Justice includes the need for restitution to be measured out to the one offended by the crime and justice needs to be exercised in such a way that it is a preventative measure in the larger society — that others, who might be considering said crimes of their own, would be turned back to walk on the straight and narrow path.
On the other hand, mercy is usually thought of as one being pardoned from either part or all of the demands of the law. The dictionary ordinarily defines mercy in terms of leniency, forgiveness, and clemency. If we are the accused, we usually think of mercy in terms of a reduced or forgiven judgment.
The funny thing has to do with the balance between them. A good and honorable judge is one that sees that the law is satisfied and obeyed. Yet, if he always punishes crime to the fullest extent of the law, without showing any mercy, he is considered ruthless and domineering. On the other hand, if a judge is always showing mercy, we consider him lax and maybe even corrupt and call for his resignation.
A middle ground can be reached in the American society because the law permits for a range in the sentencing. That way a first offender can be charged differently than a repeat offender and so that the judge may take into account the sincerity of an offender’s repentance. And so, the balance is struck and judges are held accountable by the voters in many cases.
The problem, though, when you take this idea and extend it from human experience to eternal things is that God, as judge, is not dealing with first-time offenders. We have been sinning since the day we were born — indeed, even since the day we were conceived! Further, God is not dealing with those who are likely not to sin again. We are repeat sinners and habitual sinners who will struggle with sin all of the days of our earthly life. And, God is not dealing with those who only pose a slight threat to society — our sin offends others and tempts others into sin themselves. And, even further than that, God is also dealing with those who have inherited the guilt of their first-parents’ sin. The only righteous punishment is infinite and eternal damnation in the fires of Hell. There is no range in sentencing possible.
So here, it would seem that with respect to the children of Adam, that God (as Judge) has his hands tied. Mercy cannot be accomplished simply as a matter of appealing to a range of sentencing possible within the law. Mercy becomes incompatible with justice.
Yet, is God not a merciful God? Indeed, scripture tells us that he chooses to have mercy on some (Exodus 33:19; Romans 9:15). Yet, how does God maintain his justice? The just punishment of the law must get meted out on a suitable representative for those to whom God has chosen to show mercy. Indeed, he pours it out on his Son, Jesus Christ. Question 11 of the Heidelberg catechism forms the bridge between the woeful state of our fallen souls and the redeeming work accomplished by Christ — the way that God fulfills both the demands of the Law and his choice to show mercy to some — more specifically to those whom he has elected in Christ from the foundations of the World (Ephesians 1:4-5).