The first Question in the Summa is "The Nature and Extent of Sacred Doctrine." Thomas uses the term "sacred doctrine" and "sacred science" to mean what we would normally call "theology." Since the Summa is a book about theology, Thomas starts by exploring what theology actually is. The first Article for this Question asks whether it is even necessary? The title of the Article is "whether, besides philosophy, any further doctrine is required?" If we were to translate this into modern terms, we would ask, "is there any source of knowledge beyond what science gives us?"
This is an incredibly relevant question today, and St. Thomas addressed it over 700 years ago. It's true that he didn't have access to the modern science we practice today, but his definition of "philosophy" is that which is known through reason, and this would include modern science.
Thomas' answer is based on the idea that the purpose of human life is directed towards God, who exceeds the grasp of human reason. "Hence it was necessary for the salvation of man that certain truths which exceed human reason should be made known to him by divine revelation" (I,1,1b). If there is an infinite transcendant God like the God of Christianity, and the purpose of human life is directed towards him, then in order to know how to reach God, we would need information from God that is beyond our ability to figure out on our own. In other words, God is a being so high above us that he must stoop down and tell us about himself if we are to know him.
Now Thomas does admit that there are "truths about God which human reason could have discovered," (indeed he will demonstrate some of those truths later) but it was still necessary for God to reveal them to us because only a few people would have the ability to figure it out on their own, and it would take a long time, and they would make mistakes. So even in the case of these truths about God that are accessible to human reason, God reveals them so that they can be accurately known to all.
Of course, most of the people who today say that modern science tells us everything we need to know don't believe that there is a God, or that the goal of human existence is to know God. Therefore, they would not be convinced by this argument, which assumes God's existence. However, Thomas will address this issue later when he demonstrates what we can know about God through human reason. If he can use human reason to show that there is a God and that knowing him is the ultimate goal of human existence, then he will have support for the argument made in this article. Many people make the mistake of looking at one of St. Thomas' arguments in the Summa in isolation, but that approach misses the interrelated structure of his work. I only hope I can capture a little bit of that in these blog posts.
Finally, I want to point out the Reply to Objection 2 in this Article. The Objection is that because philosophy deals with theological topics, there is no need for a study of theology apart from philosophy. The Reply notes that different intellectual disciplines, or "sciences" can come to the same conclusion using different means. "For the astronomer and the physicist both may prove the same conclusion: that the earth, for instance, is round: the astronomer by means of mathematics (i.e. abstracting from matter), but the physicist by means of matter itself" (I,1,1r2). Therefore, it is ok if philosophy comes to some of the same conclusions that are reached through theological study of divine revelation.
There are two things that I want to point out about this Reply. The first is that people in the Middle Ages knew the Earth was round. The second is that Thomas believes that there is one reality, and that different methods of investigating that reality should eventually lead to the same truth. John Polkinghorne makes the same point in his book, One World: The Interaction of Science and Theology, where he says there should ultimately be no conflict between modern science and theology if they are done rightly because they are two ways of seeking the truth of the one world we inhabit.