St Thomas Aquinas’s logic is impeccable, and fits with the rest of the systematic theology (if such an anachronism can be made to apply to the Summa Theologiae) he wrote, including the nature of God he demonstrated: it is my poor presentation of an out-of-context paraphrased quotation that made it seem not so. An earlier poster replied with the referenced sections and questions and objections in the Summa Theologiae; I recommend reading them (although, much of it may not make great sense, unless you understand a bit of the Angelic Doctor’s thought, and on top of that a good deal of Aristotelian philosophy, as his doctrinal conclusions derive from the nature of God that he demonstrated - which, for this purpose, can be stated as perfect justice, perfect mercy [the argument from Pure Act, “perfect everything”], and the reconciliation of those in human thought - and, no one person, nor all of humanity, can grasp the complete nature of God, but only parts of it, like the Jain/Hindu metaphor* of the three blind men and an elephant).
There’s no doubt that St Thomas is not infallible: as far as I can tell, he would have (or did) side with those who rejected the immaculate conception (against Duns Scotus, and their two respective schools, which became arch-nemeses; Scotism defending the Immaculate Conception, Thomism not), so at least one of his views was in error.
However, my point, drawing from St Thomas, was to answer the question, “how can a parent be happy if a child is in Hell?”; St Thomas answers this question well, even if he does not answer it perfectly, or in the only way: however, his answer is a very neat, elegant, logical and self-consistent (for both God’s nature, St Thomas’s thought, and humanity’s destiny) defense of the matter.
Hindu Version (Ramakrishna): said:
“A number of blind men came to an elephant. Somebody told them that it was an elephant. The blind men asked, ‘What is the elephant like?’ and they began to touch its body. One of them said: ‘It is like a pillar.’ This blind man had only touched its leg. Another man said, ‘The elephant is like a husking basket.’ This person had only touched its ears. Similarly, he who touched its trunk or its belly talked of it differently. In the same way, he who has seen the Lord in a particular way limits the Lord to that alone and thinks that He is nothing else.”
Jain Version (Anekantavada) said:
“A group of blind men heard that a strange animal, called an elephant, had been brought to the town, but none of them were aware of its shape and form. Out of curiosity, they said: “We must inspect and know it by touch, of which we are capable”. So, they sought it out, and when they found it they groped about it. In the case of the first person, whose hand landed on the trunk, said “This being is like a drain pipe”. For another one whose hand reached its ear, it seemed like a kind of fan. As for another person, whose hand was upon its leg, said, “I perceive the shape of the elephant to be like a pillar”. And in the case of the one who placed his hand upon its back said, “Indeed, this elephant is like a throne”. Now, each of these presented a true aspect when he related what he had gained from experiencing the elephant. None of them had strayed from the true description of the elephant. Yet they fell short of fathoming the true appearance of the elephant.”
Thus a man can focus on mercy to the exclusion of justice, or on justice to the exclusion of mercy.