So, what's the problem? Well, to put it simply, mercy is not justice. Justice, you see, is getting what you deserve. Mercy is not getting what you deserve. Mercy, then, stands in direct opposition to justice. Now, for you and me that's not an issue. We can choose to be merciful and forego justice, or we can choose to be just and forego mercy. Both are actually good. Justice is good. Mercy is good. No problem. But we are not defined by either one. Neither justice nor mercy are part of the basic definition of what it means to be "human". A human being may be just or may be merciful or may be neither and it doesn't make him or her any more or less human.
God, on the other hand, is defined by these (and other) attributes. He is good not because He meets some external standard of good, but because He defines good and is, therefore, defined by it. He is just. Abraham asked the ultimate rhetorical question, "Will not the Judge of all the earth deal justly?" (Gen 18:25). Yes, of course! It's what He does! But, wait! If God is defined by justice and mercy is not justice -- is in direct opposition to justice -- then how can God be just and merciful? Ah! That's the problem of mercy.
Some people are perfectly happy with this imbalance. They would like to rejoice in God's mercy. "That's great!" They don't want to look at the conflict. They don't want to see the problem. But God is not so shortsighted. He recognized the problem and solved it. He didn't simply redefine Himself -- "no longer just" -- but He managed to find a way to be both just and merciful. How?
Apart from the Law the righteousness of God has been manifested, being witnessed by the Law and the Prophets, even the righteousness of God through faith in Jesus Christ for all those who believe; for there is no distinction; for all have sinned and fall short of the glory of God, being justified as a gift by His grace through the redemption which is in Christ Jesus; whom God displayed publicly as a propitiation in His blood through faith. This was to demonstrate His righteousness, because in the forbearance of God He passed over the sins previously committed; for the demonstration, I say, of His righteousness at the present time, that He might be just and the justifier of the one who has faith in Jesus (Rom 3:21-26).Problem: "All have sinned and fall short of the glory of God." Big problem. You see, a Just God is required to respond to this condition with death (Rom 6:23). And not merely death -- eternal separation from God. Eternal death. That's what justice demands. Mercy would like to forgive this debt, but justice cannot. So, in His grace, God sent His Son to pay in full the debt we owe. He sent His Son to be "displayed publicly as a propitiation in His blood." The debt was owed, and forgiving the debt would be merciful, but not just. So God paid the debt Himself in His Son and became "just and the justifier of the one who has faith in Jesus."
The problem of mercy is that it's not justice. The problem with that is that God is just. So marvelous is our God that He planned and executed the plan to be just and justifier, just and merciful. Our Advocate when we sin is the One who paid our debt (1 John 2:1-2). Truly Good News! And until you understand the problem of mercy, you don't really get the marvel of God's justice and mercy. "Oh, the depth of the riches both of the wisdom and knowledge of God! How unsearchable are His judgments and unfathomable His ways!" (Rom 11:33).