I wrote this post a while back, about the Justice and Mercy of God. I thought it would be fitting to link it today, again, for Divine Mercy Sunday. I would also like to give some additional thoughts about the subject in today's post. So here are some other reasons why we ought not to consider God's Mercy and Justice as mutually opposed.
First, St. Thomas says that rather than being strictly opposed to Justice, God's Mercy actually goes beyond Justice, rewarding with more than what Justice demands. St. Thomas uses the analogy of a man, owing another man one hundred pieces of money, but by an act of mercy, pays the man two hundred. This mercy is not opposed to justice, which demands one hundred; indeed, this act of mercy more than satisfies the demands of justice, for the man is payed his one hundred, and then some. So mercy is not opposed to justice, but in fact goes beyond it, giving more than what justice by itself would give; and in so doing justice is more than satisfied. So God in His Mercy rewards the saints with much more than, by justice alone, they would have merited by their own powers. But in so doing, He doesn't go against justice, but completes, indeed, more than completes it. I think this is probably one way to interpret what St. James means when he says "Mercy triumphs over judgment" (James 2:13). Mercy is a higher standard than justice. Thus, in meeting the standard of mercy, one soars above the standard of justice, going beyond it.
What, then, of those who are damned to hell? Do they experience God's Mercy? If Mercy goes beyond justice, does it then punish the damned more than justice does? Well, yes, St. Thomas says, God's Mercy is in hell; but it obviously an absurd thought that Mercy would punish the damned more than Justice. Rather, St. Thomas says that even the damned in hell are not punished as much as they deserve to be punished. So how is this so, if Mercy goes beyond Justice? Let me explain: By justice, the damned deserve some sort of evil; and evil is a lack of the good. But God in His Mercy, as unbelievable as this may seem, allows them a greater good than the very little which they have merited by their sins. Mercy gives a good which exceeds the good which justice gives; and justice gives very little good to the damned. Mercy gives more; and thus, in a strange way, it meets and goes beyond justice. This might seem to be a strange way of looking at it, granted; but the key here is to remember that justice renders what belongs to a person, and conversely takes away what does not belong. Now, when a person commits a sin, we normally say that he therefore deserves an evil; but we would hardly say that this evil belongs to him. Rather, to say that he deserves an evil is the equivalent of saying that he deserves something good to be taken away from him, something which once belonged to him, but, by his sin, now no longer belongs to him. This is because, again, evil is defined as the absence of the good. Furthermore, in taking away from man a good which no longer belongs to him, justice must still leave some good remaining. This is the small bit of good which still belongs to the sinner, after having lost his claim to other goods, by sinning. Justice allows this small bit of good to the sinner, thus rendering to him what belongs to him. Now, in hell, the damned are reduced by justice almost to the very lowest level of their ontological goodness; practically the only goodness they would have now is their existence; all other goods they would now lack. Justice has allowed them their existence, and only their existence, in reward for the very little they have done in life. All other goods do not belong to them; hence the immense and unimaginable pain which they must undergo. But enter Mercy, which gives them some small good, beyond the mere good of existence to which justice would have reduced them; by this good of Mercy, the damned do not in fact suffer quite as much as they deserve - even if, indeed, their pains are still immense and unimaginable. Mercy has given them a good beyond the good of Justice; and St. Thomas' principle is shown to apply here as well.
Again, at first this is a strange way of looking at hell, and we may not normally think that way about the concept of justice. But once we really consider the matter deeply, I think it works quite well, given the notions of evil as a privation of the good, and justice as rendering what belongs, etc.
Now, leaving aside that aspect of the question, there is something else that St. Thomas tells us which is very interesting. Yes, it is granted, Justice and Mercy in God are both infinite and perfect. However, St. Thomas says, Justice is nonetheless founded upon Mercy, being secondary to it. How is this so? Justice presupposes an order of nature which is found in things, in creatures, in creation. There is an order of goods, and justice distributes these goods to creatures in due proportion, according to what belongs to them, etc. Justice presupposes something already existing in creation, for it gives according to what, by nature or by merit, belongs to creatures. Now, where did the nature of creatures come from? This nature determines that certain things belong to them, and thus justice gives it to them. Where did the power to merit come from? This again determines that certain things belong to the creatures meriting, and thus justice must give it to them. But there is always the presupposition of something else here, namely the nature or the merit or any other factor, according to which certain goods belong to these creatures, and justice gives it to them. But where did these presupposed principles come from in the first place? From the Goodness of God, Who gave these things to creatures of His own free will, not because there was yet another presupposed order according to which He must have given these things. In other words, He gave us our existence, our nature, our power to merit, etc., purely out of His Mercy. We didn't deserve to exist, to be who we are, to be able to do what we can do, and all these things; and thus we didn't deserve to be able to deserve anything at all. But neither were we undeserving of any of these good things. Indeed, it is quite unintelligible to even speak of deserving anything, without first presupposing these things - our existence, nature, abilities, etc. - because these things are the subject-matter, as it were, of justice itself. Thus, we exist and have all these things purely because of the Mercy of God. And because of it, we are able to claim certain things as our own, as belonging to us; and Justice builds from there, coming into play only once the Goodness of God, i.e. His Mercy, has done its first work. Justice therefore presupposes Mercy, and is founded upon it.
Understand that this does absolutely nothing to establish a contradiction between Mercy or Justice in God. On the contrary, it only shows how both His Mercy and His Justice are fully at work. But Mercy triumphs over Justice nonetheless, yet in a way which does not contradict it.
Anyway... Those are some of my thoughts for Divine Mercy Sunday. I suppose that doesn't really have that much to do, directly, with St. Faustina's beautiful devotions. For that, I'll recommend the following sermon, from one of the fine priests of the Fraternity of St. Peter (FSSP). This sermon recounts the history and the spiritual significance of the Divine Mercy devotion. It's very good. Do take a listen, and enjoy.