Below is an extreme abridgment of Brian Davies's acclaimed work, The Thought of Thomas Aquinas. A sentence has been selected from each of its 17 chapters. Interested readers are referred to Davies's lucid 376-page work.
Aquinas is not content simply to say that God is; he wants to explore the divine in as many ways as possible. Creation by God (the act of creating) is the making of something from nothing. And whatever God is, he is not dependent on anything for his existence. Further, what we say of God can be literally true, though the full reality signified by our words defies our comprehension.
God does not creatively will evil – all he wills is good; and he can only be said to will evil in the sense of permitting it, not in the sense of causing it directly. Aquinas believes that God is causally at work in the entire history of his created order, that he is absent from nothing, and that everything that happens is an expression of his will. The one God exists undivided as the simple source of all created perfection and existence.
Aquinas' philosophical case for ascribing will to God rests on his view of God's knowledge or understanding; and he develops it by drawing on the conclusion that God is wholly immutable. Providence governs all, but everything does not happen in accordance with natural necessity, and we need to allow for human freedom; yet even human freedom falls within the scope of providence since God works in everything.
For Aquinas, the heart of Christian teaching is the doctrine of the Trinity, which is the first specifically Christian topic he turns to in the Summa theologiae. We are embodied souls; and one of the things this means for him is that we have emotions or, as he calls them, passiones animae. Nothing short of God can satisfy people completely. Aquinas' position is that the Trinity makes us divine since God, who is Father, Son, and Holy Spirit, brings us to the final or ultimate good or end of rational creatures, which is nothing less than God himself. Aquinas asserts that the theological virtues are faith, hope, and charity; these, he says, are the means by which we come to God by grace as opposed to nature.
In his Christology, Aquinas conceives of Christ as the definitive means by which creatures who have come from God return to their source. Christ was both a priest and a victim, and his work bore the character of sacrifice. By means of the sacraments, we live in Christ and he lives in us.