What do Systematic Theologians talk about?
The short answer is that they talk about exactly the same things as Biblical Theologians. As we have already noted, however, while Biblical Theology traces the progressive revelation of the divine mind, pausing over the contribution of each individual era and each individual biblical writer, Systematic Theology assumes that salvation-history has come to an end and that in the Scriptures we now have God's last word (for the present). Its subject-matter, then, is 'the whole counsel of God'.
Precisely for this reason, the arrangement of Systematic Theology is topical. Its typical chapter-headings are no longer 'The Mosaic Era of Revelation', 'The Theology of the Psalter' or 'The Theology of Luke', but the great themes of the Bible. It sets out to ascertain what Scripture has to say on these topics, collates all the relevant passages, listens to them reverently, exegetes them rigorously and thus puts itself in a position where it can finally declare, 'This is what God has revealed on these great themes.'
It has to be stressed that these are biblical themes. Systematic Theology as such has no interest in the questions put by philosophy and can offer no theological answer to them. This is not to say that there is no overlap between the two sets of questions; or that philosophical questions themselves are either uninteresting or unimportant; or that the theologian can always afford to ignore the objections which philosophy raises against his theological formulations.
Nor should the theologian feel it beneath his dignity to clarify Christian doctrine by means of terms with a previous history in philosophy. Key theological words such as 'substance' and 'essence' were borrowed from philosophy; and we are free to put the terminology of Martin Buber's classic, I and Thou, to good theological use even while not endorsing his views on man's experience of God (or even pretending to understand them!).
Nevertheless, the Christian scriptures have no voice on, for example, the debates between determinists and libertarians, or idealists and empiricists; and because Scripture is silent, theology is silent. Theologians may pronounce on such issues. Theology cannot. The Christian philosopher may, of course, address them, but he must do so on philosophical, not theological, grounds; and the Christian theologian must recognise that whatever his personal views on such questions they cannot claim the authority of divine revelation. We cannot put a theological imprimatur on Platonism, Aristotelianism or even Scottish Realism (nor, of course, can we expect a philosophical imprimatur on our theology, or set out to establish Christian doctrine from purely philosophical presuppositions). Theologians, including Systematic Theologians, must limit themselves to their own domain, the great doctrines of the Christian faith; and these doctrines are never products of our own philosophical acumen, but 'mysteries' revealed to us in the Scriptures.
Do we have to systematise?
But not, of course, in an organised and systematic form. There is, for example, no biblical book bearing the title, 'Christology' or any Pauline treatise on the atonement. Would it not be better, then, to refrain from systematising and simply reproduce biblical statements as we find them?
In reality, not even the most dogged bibliolatry can do that. The human mind systematises instinctively because it never sees facts in isolation. It sees everything in relation to time and space (the When? and the Where? of all that we observe); it sees actions as good or bad; and it sees events not in isolation, but in their connections (before or after; as causes, consequences or accidents).
Truth is, we cannot think without systematising. 'Systematisation,' wrote Warfield, 'is only a part of the inescapable effort of the intelligence to comprehend the facts presented to it'; and in accordance with this he remarks, 'If we know so much as two facts concerning God, the human mind is incapable of holding these facts apart'.
This is why the Christian cannot avoid being a systematic theologian. For example, countless biblical passages speak of Christ as divine; countless others speak of him as human. Our minds, by very force of nature, want to systematise these facts, and the result is the two-nature doctrine of the person of Christ.
But just besides these facts lie two other facts: God is one, and yet Jesus, as well as the Father, is God. This immediately poses a question to Christian piety. Are we to worship Jesus (God the Son) in the same way as we worship God the Father? But it also poses a question to the Christian mind. How can God be simultaneously One and Two (and Three, when we remember the Holy Spirit)? It is these facts which are systematised - and demand to be systematised - in the Christian doctrine of the trinity.
Every Bible student engages to some extent in this sort of systematising, collating and harmonising scripture. Systematic Theologians merely do it with greater rigour (and enthusiasm!).
No mandatory arrangement
We have to remember, however, that while the Bible reveals the great doctrines of the faith it says nothing about the order in which they are to be systematised. There is no mandatory arrangement. Calvin's Institutes followed the order of the Apostles' Creed. Thomas Chalmers, began with, 'The Disease for which the Gospel Remedy is Provided', went on to discuss 'The Nature of the Gospel Remedy' and concluded with 'The Extent of the Gospel Remedy'. And John Frame boldly treats 'The Doctrine of the Christian Life' after 'The Doctrine of the Last Things'.
Barth was fond of quoting the dictum, methodus est arbitrarius, and while that may be overstating the case, the order in which Systematic Theology arranges its material is a matter of personal choice, influenced by such factors as historical context (the issues being debated at any particular time), clarity of presentation, and the need for effective teaching (the original driving-force behind Scholasticism).
But this requires two qualifications.
First, it is fascinating that the arrangement followed by most Reformed theologians (Theology [the doctrine of God], Anthropology, Christology, Soteriology, Eschatology) conforms so closely to the historical time-line: 'in the beginning, God', followed by creation, the fall, the incarnation, the cross, the mission of the Spirit and, finally glorification.
Second, although there is no canonical order, questions of order have sometimes generated vigorous debate. Should, for example, the doctrine of the 'one God' (De Deo) be treated before the doctrine of the trinity? And where should we place the doctrine of the divine decrees? Should we place it before the doctrine of creation (like Turretin, Hodge, Berkhof and the Westminster Confession) and thus expose ourselves to the charge of giving it undue prominence? Or should we, like Calvin postpone it to Book III, and discuss it only in connection with the mystery that 'the covenant is not preached equally among all men, and among those to whom it is preached, it does not gain the same acceptance either constantly or in equal degree.' (Institutes III:XXI, 1).
All doctrines interconnected
Finally, Systematic Theologians are not interested merely in arranging biblical topics in a satisfactory order. They are also interested in the connections between them. The underlying premise here is that revealed truth is one, and that its various elements are organically related (all centred ultimately on Christ). Each presupposes and reinforces the other; and, conversely, error in one part will cause dislocation everywhere else. For example, denial of the deity of Christ dislocates everything else. But scarcely less important is our doctrine of sin. It may lack the magnificence of such doctrines as the incarnation and the atonement, but to minimise the gravity of sin is to throw everything else out of joint, including our view of the need for a divine redeemer, vicarious satisfaction and invincible sovereign grace.
Other doctrines similarly reinforce each other. For example, the fact that all the persons of the Trinity are 'the same in substance' creates a strong presumption against any idea of subordination; the unity between the Son and the Holy Spirit creates a strong a priori case that union with Christ invariably involves baptism in his Spirit; and the doctrine of Christian Liberty draws strong support from the doctrine of adoption. If we are God's children we cannot let ourselves be subjected to the 'doctrines and commandments of men' (Westminster Confession, 20:2).
It is because of this organic connectedness and interdependence of the various biblical doctrines that monographs on one particular theme often end up covering almost the whole of Christian truth. For example, Barth's two volumes on The Doctrine of the Word of God contain not only his treatment of Revelation, but also his expositions of the doctrine of the Trinity, the Incarnation and 'The Outpouring of the Holy Spirit'; and John Owen's Discourse on the Holy Spirit contains almost as much Christology as Pneumatology (not to mention its extensive treatment of Sanctification).
However, there are also some doctrines which exist in a state of apparent tension. How can God be simultaneously one and three, or Christ be simultaneously God and man? How can God foreordain all things and yet hold each one of us responsible for our actions (Rom. 9:19)? The temptation here is to suppress one or other horns of the apparent dilemma. The safe course is to do justice to both, allowing each to stand on its own independent biblical evidence and recognising that while scripture clearly teaches both, it offers no theological answer to the question, How can they both be true? And where there is no theological answer, philosophy should fear to tread.
Donald Macleod is Professor of Systematic Theology at the Free Church of Scotland College, Edinburgh, Scotland. He is the author of A Faith To Live By (Christian Focus, 1998), The Person of Christ (IVP, 1998) and Christ Crucified (IVP, 2014)
 Martin Buber, I and Thou, 2nd edition, trans, Ronald Gregor Smith (Edinburgh: T&T Clark, 1959).
 B.B. Warfield, Studies in Theology (Grand Rapids: Baker, 1991), p. 95
 John Owen, Works, vol. III (reprinted London: Banner of Truth, 1966)