At the head of each of the four books of the Summa contra gentiles, St. Thomas places a verse from Scripture that expresses his plan for the book.
Book I. On God. My mouth shall meditate truth, and my lips shall hate impiety (Prov 8:7).
Wherefore the twofold office of the wise man is fittingly declared from the mouth of wisdom, in the words above quoted; namely, to meditate and publish the divine truth, which antonomastically is the truth, as signified by the words: my mouth shall meditate truth; and to refute the error contrary to truth, as signified by the words: and my lips shall hate impiety, by which is denoted falsehood opposed to divine truth. (SCG I, 1)
Book II. On Creation. I meditated on all thy operations: I meditated upon the works of thy hands (Ps 142:5).
Now the operation of a thing is twofold, as the Philosopher teaches; one that abides in the very worker and is a perfection of the worker himself, such as to sense, to understand, and to will; and another that passes into an outward thing, and is a perfection of the thing made that results from it, such as to heat, to cut, and to build… Of the former operation of God we have already spoken in the foregoing book, where we treated of the divine knowledge and will. Wherefore in order to complete our treatise of the divine truth, it remains for us to treat of the latter operation, whereby, to wit, things are made and governed by God. We may gather this order from the words quoted above. For first he speaks of meditation on the first kind of operation, when he says: I meditated on all thy operations, so that we refer operation to the divine intelligence and will. Then he refers to meditation on God‘s works when he says, and I meditated on the works of thy hands, so that by the works of his hands we understand heaven and earth, and all that is brought into being by God, as the handiwork produced by a craftsman. (SCG II, 1)
Book III. On Providence. God is a great Lord and a great king above all gods. For the Lord will not reject his people. For in his hands are all the ends of the earth and the heights of the mountains are his. For the sea is his and he made it, and his hands formed the dry land (Ps 94:3-5).
Wherefore, the Psalmist, filled with the divine spirit, in order to give us an illustration of the divine government, first describes to us the perfection of the supreme governor—as to his nature when he says, God; as to his power, when he says, a great Lord, implying that he needs no one for his power to produce its effect: as to his authority, when he says, a great king above all gods, since, although there be many rulers, yet are all subject to his rule. Secondly, he describes to us the manner of this government. As regards intellectual beings, which, if they submit to his rule, receive from him their last end which is himself; wherefore he says, for the Lord will not reject his people. As regards things corruptible which, albeit at times they wander from their proper mode of action, never escape the power of the supreme ruler, he says, for in his hands are all the ends of the earth. And as regards the heavenly bodies, which transcend the highest summits of the earth, that is of corruptible bodies, and always maintain the order of the divine government, he says, and the heights of the mountains are his. Thirdly, he assigns the reason of this universal government, for the things that God made must needs be governed by him. To this he refers when he says, for the sea is his, etc. (SCG III, 1)
Book IV. On Salvation. Behold, these things are said of his ways in part: and seeing we have heard scarce a little drop of his word, who shall be able to behold the thunder of his greatness? (Job 26:14).
Hence man‘s knowledge of divine things is threefold. The first is when man, by the natural light of reason, rises through creatures to the knowledge of God. The second is when the divine truth which surpasses the human intelligence comes down to us by revelation, yet not as shown to him that he may see it, but as expressed in words so that he may hear it. The third is when the human mind is raised to the perfect intuition of things revealed. This threefold knowledge is indicated by the words of Job quoted above. The words, these things are said of his ways in part refer to the knowledge in which our intellect rises to the knowledge of God by the way of creatures. And because we know these ways but imperfectly, he rightly adds in part: since we know in part, as the Apostle says (1 Cor 13:9).
The words that follow, and seeing we have heard scarce a little drop of his word, refer to the second knowledge, wherein divine things are revealed to our belief by way of speech: because faith, as it is said, is by hearing, and hearing is by the word of Christ (Rom 10:17), of which it is also said (John 17:17): sanctify them in truth. Thy word is truth. Wherefore, since the revealed truth in divine things is offered not to our sight but to our belief, he rightly says we have heard. And whereas this imperfect knowledge flows from that perfect knowledge whereby the divine truth is seen in itself, when revealed to us by God by means of the angels who see the face of the Father, the expression drop is appropriate: hence it is said: in that day the mountains shall drop down sweetness (Joel 3:18). But since not all the mysteries which the angels and blessed know through seeing them in the first truth are revealed to us, but only a certain few, he says pointedly, a little. For it is said: who shall magnify him as he is from the beginning? There are many things hidden from us, that are greater than these: for we have seen but a few of his works (Sir 43:35-36). Again the Lord said to his disciples: I have yet many things to say to you: but you cannot bear them now (John 16:12). Moreover these few things that are revealed to us are proposed to us figuratively and obscurely, so that only the studious can succeed in understanding them, while others revere them as things occult, and so that unbelievers are unable to deride them. Hence the Apostle says: we see now through a glass in a dark manner (1 Cor 13:12); wherefore Job adds significantly the word scarce, to indicate difficulty.
When he goes on to say, who shall be able to behold the thunder of his greatness? he is referring to the third knowledge, whereby the first truth shall be known as an object not of belief but of vision, for we shall see him as he is (1 John 3:2); wherefore he says, behold. Nor shall a small portion of the divine mysteries be perceived, but the divine majesty itself shall be seen, and the entire perfection of good things: hence the Lord said to Moses: I will show thee all good (Exod 33:19); wherefore he says rightly, greatness. Nor will the truth be revealed to man obscurely, but made clearly manifest: wherefore our Lord said to his disciples: the hour cometh when I will no more speak to you in proverbs, but will show you plainly of the Father (John 16:25); hence the word thunder is significant as indicating manifestation.
Now the passage quoted is suitable to our purpose: because hitherto we have spoken of divine things, in as much as natural reason is able to arrive at the knowledge of them through creatures; imperfectly however and as far as its own capacity allows, so that we can say with Job: behold, these things are said of his ways in part. It remains then for us to speak of those things that God has proposed to us to be believed, and which surpass the human intelligence. (SCG IV, 1)