It can be easy to think God is sometimes angry and sometimes merciful.
And it is true that sometimes His mercy may be more apparent while at other times His judgment seems to be the forefront, as St. Thomas Aquinas says.
But in all His works, God’s justice is never without His mercy and vice versa, according to Aquinas, who devotes Article 4 of Question 21 in the Summa Theologica to this truth.
This should be both a bit jarring and comforting to us. It means that even when it seems that God is at His angriest with us or when we seem most deserving of His judgment and condemnation that His mercy is too far away either. But it also means that in His mercy doesn’t occur without also making things right. Put another way: there is no mercy without repentance and renouncing of sin and self.
But why do justice and mercy always go together in God?
Our answer starts with the traditional doctrine of the absolute simplicity of God.
This is best explained by way of contrast with creatures. We’re certainly not simple. To begin with, we have bodies and souls. God, on the other hand, is pure spirit. (The Incarnation did not change God. Rather, in the Incarnation God assumed human nature, changing humanity.)
But the differences do not stop here. Even in the realm of the immaterial, there is composition within us where there is none in God. Some of us have wisdom, prudence, and courage. But not all of us do. These are characteristics that we acquire. In the very odd-sounding terminology of Thomism we would say that wisdom was an ‘accident’—that is something added on to our being or substance.
In creatures, accidents, or characteristics are something we gain or lose. A white table can become stained with use. Wine can sour. Wise men can become fools and the powerful can become weak.
It should be apparent now God does not have any ‘characteristics’ in the sense of ‘accidents’ that are added on. God cannot lose His wisdom and power; otherwise, He’d be less than God. God, the creator of the universe, cannot lose His power. What could possibly take it away from Him? Likewise, as one who is all-knowing how could God lose His wisdom?
Moreover, if He acquired power and wisdom then He was once not God and but became so, acquiring them from somewhere or someone else. And that would put us deep into rabbit hole of heresy and pantheism, believing in another god as the source of power and wisdom. (For more on why God does not have ‘accidents’ see Aquinas here.)
Thus, the very concept of an all-knowing, all-powerful God requires that He be every ‘characteristic’ He ‘has.’ Here is the example St. Augustine offers, speaking directly of God the Father’s greatness and wisdom:
[W]hy is He not also the Father of His own greatness by which He is great, and of His own goodness by which He is good, and of His own justice by which He is just, and whatever else there is?
In other words, Augustine is reminding us that God is the source of His own greatness, goodness, justice, wisdom, and power. Because these things are not acquired from anywhere else they are God’s very being. If this is true, then God is His own attributes, at least those we could positively affirm of Him. He is His own goodness. And He is His own greatness, wisdom, and power because they are indistinguishable from His being. As Augustine puts it,
Or if all these things are understood, although under more names than one, to be in the same wisdom and power, so that that is greatness which is power, that is goodness which is wisdom, and that again is wisdom which is power, as we have already argued; then let us remember, that when I mention any one of these, I am to be taken as if I mentioned all.
[I]t is not one thing to Him to be great and another to be God …For as He is great, only with that greatness which He begot, so also He is, only with that essence which He begot; because it is not one thing to Him to be, and another to be great. Is He therefore the Father of His own essence, in the same way as He is the Father of His own greatness, as He is the Father of His own power and wisdom? Since His greatness is the same as His power, and His essence the same as His greatness.
This brings us back to our present topic, regarding God’s justice and mercy. We know that God has justice. We also know that He is merciful. Because of our understanding of who God is, thanks to Augustine and Aquinas, we also know that mercy and justice are not some added-on qualities that God got from someone else. They are intrinsic to who He is.
To paraphrase Augustine from above, it is not one thing to Him to be and another to be merciful. His essence is the same as His mercy. And likewise, His essence is the same as His justice. This helps to explain why justice and mercy—together—are present in all of God’s works.
For us it means that in the face of the injustices of the world, God’s justice endures nonetheless. It may seem like there is no justice in the world but that’s not because God has lost His justice. And conversely for sinners—that is, all of us—it means that God’s mercy is never far behind His judgment. And it means that just as there is justice and mercy in everything God does, so also is there wisdom, power, and greatness.