St Thomas wrote the Summa Contra Gentiles with the following in mind:
 And so, in the name of the divine Mercy, I have the confidence to embark upon the work of a wise man, even though this may surpass my powers, and 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 proceed against individual errors, however, is a difficult business, and this for two reasons. In the first place, it is difficult because the sacrilegious remarks of individual men who have erred are not so well known to us so that we may use what they say as the basis of proceeding to a refutation of their errors. This is, indeed, the method that the ancient Doctors of the Church used in the refutation of the errors of the Gentiles. For they could know the positions taken by the Gentiles since they themselves had been Gentiles, or at least had lived among the Gentiles and had been instructed in their teaching. In the second place, it is difficult because some of them, such as the Mohammedans and the pagans, do not agree with us in accepting the authority of any Scripture, by which they may be convinced of their error. Thus, against the Jews we are able to argue by means of the Old Testament, while against heretics we are able to argue by means of the New Testament. But the Muslims and the pagans accept neither the one nor the other. We must, therefore, have recourse to the natural reason, to which all men are forced to give their assent. However, it is true, in divine matters the natural reason has its failings. [I:2]
In short, SCG is a work of apologetics. Personally I think it’s brilliant, and when I first read it I thought it was more accessible than the Summa Theologiae—not that it is easy, but it’s not as hard.
What I particularly appreciate about this passage, though, is that he tells us that he intends to frame his arguments for the sake of his audience. I’m reminded of another favorite quotation:
Good and ill have not changed since yesteryear; nor are they one thing among Elves and Dwarves and another among Men. It is a man’s part to discern them, as much in the Golden Wood as in his own house. [The Two Towers, “The Riders of Rohan”].
Good and evil do not differ among Elves and Dwarves and Men, and the truth is one and the same thing for Jews and Christians and Muslims and pagans; the fact that they differ with each other doesn’t mean that truth is pluriform. But the authorities recognized by these different peoples aren’t the same. So it makes sense to tailor one’s argument according to the authorities that they do receive, by appealing to the OT with Jews, to the NT with heretical Christians, and to reason with Muslims and pagans. But really, reason’s force is the same for all: if a man’s premises are valid and true, and if his conclusion follows from his premises, then we are compelled to accept the truth of his argument. And this is what Aquinas intends to present in SCG: an argument for the Catholic faith which, as far as he is able and as far as the frailty of human reason permits, will compel his readers to accept his conclusions.
Some readers may recognize that St Thomas’ approach differs considerably from the presuppositional apologetics of Van Til. Aquinas would not agree with presuppositionalists that God’s existence must be presupposed.