It's not exactly the same sort of work as the Summa Theologica, of which Aquinas says, "we purpose in this book to treat of whatever belongs to the Christian religion, in such a way as may tend to the instruction of beginners." Setting aside the question of what sort of "beginner" he had in mind (probably the equivalent of a well-read graduate student today, I suppose), he sets out to explain Christian doctrine to fellow believers.
SCG is different.
I have set myself the task of making known, as far as my limited powers will allow, the truth that the Catholic faith professes, and of setting aside the errors that are opposed to it. To use the words of Hilary: “I am aware that I owe this to God as the chief duty of my life, that my every word and sense may speak of Him” [I-2]In other words, SCG is intended as a work of apologetics, and so he does not presume that his audience is Christian. That might seem to be mere posturing at first to some of us today, because we might have the idea that 13th century Europe was so completely Christian as to make an apologetic work for non-Christians nothing more than a purely academic exercise. In fact, however, it was only during Aquinas' lifetime that Muslim rule over much of Spain was overturned, and there were of course others who could likewise profit from his labors (to say nothing of the benefit that Christians might enjoy).
A word ought to be said about the nature of the argument he presents. The Catholic Faith is rational, but it is not rationalistic. That is, nothing that we believe is irrational, or contrary to reason, although there are certainly things that we believe which we could not apprehend solely on the basis of reason. Some things must be revealed, and we must simply believe them. In SCG, Aquinas sticks to presenting a rational argument for the Faith. He does not try to present arguments for things that must simply be believed as though he can prove them to be true. So when he discusses the Trinity, or the Eucharist, he doesn't pretend to present arguments that "prove" them. Rather, he presents arguments that demonstrate them to be reasonable rather than irrational. On the other hand, he is convinced that there are also many things about God and Christian truth that certainly may be rationally demonstrated, and in these cases that is precisely what he attempts, along with showing that the conclusions of his arguments are consistent with the teaching of Scripture and the Church.
I guess another thing worth saying - and I'm not the first to say it by a country mile, and others say it much better than I will - is that there is a ridiculous conceit in our day that true things are empirically verifiable - that the only "real" truths are those of math or science. This is total malarkey. There are many things that we understand to be true by use of reason that may not be subject to empirical observation but which are nonetheless true. St. Thomas does not suffer from the burden of having to deal with this empiricist delusion, and we would do well to shake it off ourselves.