So what good reasons are there for doing systematic theology (ST)? We shall stick with this definition of ST: that it is the attempt to answer the question, what does the whole Bible say on this biblical subject? It is a synthesising, harmonising and summarising discipline using the biblical materials provided by exegesis and biblical theology.
I am making several assumptions at this stage: (i) that God exists; (ii) that he has revealed himself in many ways but, supremely, in writing in the Scriptures and personally in his Son ; (iii) that his revelation has been given in the context of covenantal relationship with his human creatures.
The first good reason for doing ST is that the nature of God demands it. The Lord is one God , perfectly self-consistent, and of one mind. Thus, presupposing as we must do to begin theology that God has spoken, we can further assume that his self-revelation will be self-consistent. To examine that revelation as thoroughly as we can to pursue its inner harmony is a duty of the covenant creature. The use of our minds made, as the whole person is made, in God's image, to pursue the mind of our Maker in his revelation is part of loving him with all our mind. To seek out the consisency of that revelation is to pursue his mind as far as we can. As Douglas Kelly puts it in the early pages of his Systematic Theology, it is to put questions to God's revelation so that he will reveal himself further.
So doing we see the parts of God's revealed will in relation to each other and in relation to the whole. We think the thoughts of God after him. Warfield's illustration is not perfect but helpful nonetheless: if exegesis gathers together individual soldiers, biblical theology puts them into regiments and companies, and ST sees them as part of a complete army to be manouevred into battle.
Is this artificial? No. We do it automatically. Any Christian who witnesses to a friend to tell them 'you are a sinner, Christ died for sinners, so believe and be saved', is using ST. Any believer who wearily but gladly rests on 'the Lamb of God, slain for me' is doing so in the context of a ST. For what would the individual idea mean apart from the framework of Scripture?
By nature we are synthesisers of information, striving for unity. The challenge is to do it as thoroughly and accurately as possible. The nature of man in God's image requires ST. Our use of reason to draw deductions from Scripture is authorised so far as it does not conflict with other Scripture; complete harmony will not be possible and we shall have to content ourselves with living with paradox at many points.
Second, the character of God demands it. God is self-consistent; he is also faithful and reliable. We can trust his revelation. His trustworthiness means he tells us the truth. A lover of truth, which a lover of God will be, will want to understand truth as far as possible as a homage to his Lord.
Third, the nature of Scripture demands it. The Bible is a book from God that is not set out systematically in the sense we are using. It is in some senses a history book, tracing the story of God's purposes from creation to consummation. Why do we need more? Quite simply, so we can preach it. Scripture has been given developmentally and incrementally; the message is not explicit until at least the Christ event of the New Testament, and possibly not until the end of Revelation. The whole has to be read in the light of the last 20% or so, the Old in the light of the New. Biblical Theology is in some ways an Old Testament discipline; once Christ has come, its work is largely done; equally, ST is only just beginning here and begins to understand the whole in the light of the consummation.
If ST did not do this, we would forever be telling a story but without a punchline; we would have no gospel to preach, no demand to make in God's name of sinners, no good news to offer.
B.B.Warfield says: 'The systematic theologian is pre-eminently a preacher of the gospel and the end of his work is obviously not merely the logical arrangement of the truths which come under his hand, but the moving of men, through their power, to love God with all their hearts and their neighbour as themselves...' For this, he needs to 'be having a full, rich, deep religious experience of the great doctrines with which he deals; he needs to be living close to God'.
Moreover, the student of systematic theology, adds Warfield, must not be merely a student, 'but, like the beloved disciple himself, in every sense...a divine'. ('The Idea of Systematic Theology',p 86-7 in Studies in Theology.)
Some more reasons for ST next time, and also a response to some who think doctrine needs to be thought of differently if we are to make it useful to the church.