God’s mercy and justice are not in competition: Aquinas explains how

Does God have a split personality? We sometimes hear people speak of him as if he did. Indeed, a second century priest named Marcion actually proposed this was true. Marcion wrote a book called the Antitheses in which he placed side by side passages from each Testament that he found contradictory.

Many of the differences center on Marcion’s claim that the God of the Old Testament loved justice, while the God of the New Testament loved mercy. For example, he quotes Exodus 21—“an eye for an eye and a tooth for a tooth”—and opposes it to Luke 6—“turn the other cheek.” Marcion attempts to demonstrate that “The Creator God is judicial, harsh, and mighty in war” while “the Supreme God is gentle and simply good and excellent.”

While we know there is only one God, we do sometimes still hear people speak of “The God of the Old Testament” and “The God of the New Testament,” as if they were two different beings. How do we reconcile the sometimes differing pictures we see of God?

Jesus tells us our Father in Heaven is perfect (Matthew 5:48). God would not be God if He did not contain all perfections. Any good that we can conceive of, God possesses it completely. So, God is perfectly good, perfectly loving, all-powerful, and all-knowing, and more. And since God is perfectly simple—that is, God does not have components or parts—all perfections are one in God. God’s knowledge and power are one, as are his goodness and presence. 

Still, there are some goods, like justice and mercy, that we have a hard time picturing together. Justice is a good. It is the virtue of giving to others what they are owed, what they deserve. Mercy is also a good. We often think of mercy as relenting in punishment, or sparing another of the consequences of their actions. 

Don’t these two conflict? If we are acting with justice toward someone who has done wrong, will we not make them pay for their deeds, as a matter of retribution? If we act with mercy toward a wrong-doer, will we not release them from their sentence early, or even forego it entirely? Mercy and justice would seem to be opposites. Yet they are both goods in their own right. How can God be both perfectly merciful and perfectly just?

The answer to this question lies in the Cross, and St. Thomas Aquinas helps us to understand it.

In the Summa Theologiae (III, q. 46, a. 1, ad.3), St. Thomas affirms that God could have saved us in a way other than by the sacrifice of His Son. Though God could have restored humanity to His grace merely with a word, St. Thomas says the Cross was most fitting, because “Christ's Passion was in keeping with both His mercy and His justice.” The Passion fits with God’s justice because “satisfaction was made for the sin of the human race.” The consequences of sin are suffering and death. By his Passion and death, Christ thus saves us from sin by taking on to Himself the consequences of our sin. Christ does not merely wave away our sins, but pays for them. The satisfaction owed for our sins is made, and thus justice is done.

The Passion also fits with God’s mercy, because “man of himself could not satisfy for the sin of all human nature,” so the Son of God Himself came to give Himself to save us. Because we owed a debt we could not pay, God paid the debt for us, thus showing us His mercy. It is akin to a judge having his own son before him in court. The judge fines his son for his offense, but then takes off his judge’s robe, comes down from the bench, and pays his son’s fine for him.

The judge is just because he demands satisfaction be made for the offense, and merciful because he makes satisfaction himself. This is a greater act than merely dismissing the offense and the fine. In the same way, God shows us his mercy by paying the debt owed for our sins, rather than leaving us indebted forever. Because God saves us not by a mere command but by an act of self-sacrifice, he acts with greater mercy. As St. Thomas says:

The Cross is God’s demonstration to us that mercy is not opposed to justice. The two are not contraries. They are on the same side of the ledger. Mercy is not on the opposite side of the spectrum to justice, but rather a greater form of justice. Mercy does not lay aside justice, but surpasses it. 

St. Thomas notes that in every act of God, both mercy and justice are at work. Our very existence is in a sense an act of mercy, because we are not owed existence from God. God creates us from an outpouring of his goodness—in our very being, God gives us more than what we are owed. In forgiving our sins, He does no less. Pope Francis encapsulated this truth well when he wrote,