Irrational Man is an introduction to existentialism written in 1958. In Ch. 5, Barrett discusses the Christian sources of existentialism, including the relationship of St. Thomas Aquinas to existentialism. Certain Thomists, Etienne Gilson for example, had claimed to find existentialist themes in the Angelic Doctor. Barrett does not find much merit in this position:
A good deal of the Thomistic existentialism current nowadays looks indeed like a case of special pleading after the fact. A book like Gilson's, for example, shows so strongly the influence of Kierkegaard (albeit at work on a mind that is granitically Thomist) that it is safe to say the book could not have been written if Kierkegaard had not lived. Without Kierkegaard, indeed, Gilson would not have found in St. Thomas what he does manage to dig out, and the fact is that a good many other Thomists found quite different things before the influence of Kierkegaard made itself felt. And, to go one step further, what Gilson finds is not enough. The historicity of truth is inescapable, however perennial the problems of philosophy may be, and we should be suspicious in advance of any claim that the answer to modern problems is to be found in the thirteenth century. Granting St. Thomas' thesis of the primacy of existence and of the real distinction between existence and essence, we are still very far from an answer to those questions which have led modern thinkers like Heidegger and Sartre to a reopening of the whole subject of Being.
This passage is worth reading, by the way, for the use of the neologism "granitically" alone. But I think Barrett misses the value of reading St. Thomas with respect to existentialism. It is true that the existentialist question as we know it today is a modern problem that was not really known to St. Thomas; this is somewhat like noting that the modern problem of sin was not known to Adam and Eve before the Fall. Even if true, it doesn't follow that the primordial state has nothing to teach us now.
The philosophical "state of innocence" analogous to the Edenic primordial state is the philosophical state prior to the modern disruption between thought and existence. Prior to the modern era, thought and existence were united in the being of the philosopher. All this means is that the philosopher lived his thought and lived in his thought; or, rather, the unity of life and thought was something taken for granted.
The difference can be seen in the lives of Socrates and St. Thomas vs. the modern academic professor of philosophy. The biography of St. Thomas is inseparable from the philosophy of St. Thomas; if you knew nothing about St. Thomas's philosophy but knew the story of his life, you would be able to guess a good deal of the philosophy. Compare that with the modern professor of philosophy. If you knew the biographical facts of a particular professor, would you necessarily know whether the professor was even an atheist or a theist? Kant was perhaps the first and greatest example of the modern academic philosopher. His thought was revolutionary in the deepest senses of the word; he was deliberately embarked on a "Copernican revolution" in thought that was intended to change man forever. Yet his day to day life was the epitome of conventional respectability and regularity. It was said that he was so predictable in his habits that the housewives of Konigsberg could set their clocks by the time of his daily walks. The "form" of Kant's life did not reflect the revolutionary content of his thought. St. Thomas was something of an intellectual revolutionary in his own day, given that his philosophical master Aristotle was, at the time, viewed as a dangerous innovation (having recently been rediscovered) in a world dominated by Platonism. And St. Thomas was no tidily respectable university professor; he scandalized his noble family by committing himself to joining an order of mendicant friars - the Dominicans - who at the time were at least as disreputable among polite society as hippies or Jesus freaks are now. As Chesterton so pithily puts it - "St. Thomas would not rest until he was duly and regularly appointed a beggar." (I paraphrase from memory from his biography of St. Thomas). The "form" of St. Thomas's life followed its revolutionary content.
This isn't to say that every modern philosophy professor lives abstracted from his thought. It is only to say that we no longer take such integration for granted, and this is a real difference. Humpty Dumpty can be put back together, but our default philosophical state is that of Humpty Dumpty in pieces on the ground. Surely a thorough appreciation of the integrated existence that philosophers once possessed is worthy of study, just as Humpty Dumpty might gaze on his fellows who never fell off the wall. Suppose Humpty Dumpty wakes up from his fall and forgets what it was like to be on the wall or, worse, thinks that his broken state is his natural condition? This is the state of modern man and it was the genius of Kierkegaard to recognize the condition; and his further genius to find a way to communicate that the modern and existentially fragmented philosophical condition is not natural, but a self-inflicted fall induced by modern philosophy; and to point us back and reveal to us the true nature of the philosophers who were classical and whole.