Theology is one science by species under divine revelation, and is practical rather than merely speculative

Further, theology is one science by species, not many. For although it treats of things that are very diverse — for example, of God, of angels, of man, of virtues and vices, which seem to belong to altogether different sciences — still all those things agree in one common formal reason, under which they confront and are objected to theology. That common reason is divine revelation. For the things I have named do not fall under the theologian's consideration in any way whatever, but only insofar as they can be known through divine revelation. For example, concerning the nature, powers, and duty of man the theologian does not teach whatever can be taught, but only those things which he has learned and gathered from the word of God.

Here it must also be noted that not whatever God reveals in any manner whatever pertains to the object of theology. For God may have revealed certain things privately to many persons, which do not fall under this science. But here by revelation we understand that which is common to the whole church, and which was made for its common usefulness, and which God willed to be consigned in writing. For the theologian ought to prove his assertions not from private revelations made to this person or that, but from sacred Scripture.

Further, theology does not seem to be a science merely speculative, nor merely practical, but practical and speculative at once. And that is because theology treats of many things that belong to practical science as well as to speculative. For it contemplates God and the divine mysteries, which do not fall under action; and it also gives precepts of life, and teaches what manners are worthy of a Christian man.

Nevertheless we judge theology to be practical rather than speculative. For the whole of theology is occupied in the explanation and defense of the things contained in the Scriptures. But the teaching contained in the Scriptures, from the scope of God himself who teaches and reveals, is referred to action — namely to the love of God and of neighbor. Nor has anyone attained the scope of this teaching who does not refer whatever he has learned from the word of God to that end. For Christ affirms in (Matthew 22:37-40) that the law and the prophets hang on these two commandments: you shall love the Lord your God with all your heart and with all your soul, and your neighbor as yourself. Then Paul says in (Romans 15:4) that whatever things were written were written for our learning, that through patience and the comfort of the Scriptures we might have hope. But above all his words in (2 Timothy 3:16-17) are to be weighed: All Scripture is inspired by God, and is useful for teaching, for reproof, for correction, for discipline which is in righteousness; that the man of God may be perfect, and thoroughly furnished unto every good work.

To this belongs the fact that the Gospel is called by the same Paul a teaching according to godliness (1 Timothy 6:3). For that is for no other reason than that the chief scope of the Gospel is to form us unto godliness. Indeed those articles which seem most to pertain to contemplation — namely concerning the divinity and incarnation of the Son of God — are called by the Apostle in specie the mystery of godliness, in chapter 3 of the same epistle (1 Timothy 3:16). Add that the habit of the theological principles is faith. But true faith is a practical habit. And the habit of the principles and the habit of the conclusions are of the same genus.