Systematic theology declares that God is God: sovereign, all-powerful, all-wise, and all-knowing, ordaining and decreeing from eternity all that is; that therefore there is no unrealised possibility or potentiality in God.
Only with this kind of God is systematic theology possible. Wherever faith in the sovereignty of God weakens, so also systematic theology.
Systematic theology means, first, that it is a unified whole of what the Scripture says about God. It is the result of biblical theology, the exegesis of all the biblical data in summary and comprehensive form.
Second, it means that the sovereign God who does not change (Mal 3:6) is truly knowable, that God is unchanging and self-consistent. Rooted in Karl Barth’s belief that God and omnipotence are mutually exclusive, modern theology falters at providing a systematic theology of the God of Scripture, preferring its notion of ‘possibility’ – or change – in God, which then opens up a world of possibilities for man apart from God. God is no longer sovereign but man. To affirm any possibility outside of God and his decrees, though, is to affirm the ultimacy of chance. It follows that a systematic theology in an arbitrary universe is non sequitur—logically fallacious.
Third, it presupposes the mind of the God of Scripture, not man’s, the sovereign God and his ordination of all that is as the only ground of reasoning and proof. The alternative is brute factuality—unexplained, unrelated facts, rendering systematic theology impossible.
Fourth, it denies the notion of neutrality. No facts, thoughts, nor man – including reason itself – is neutral. All of these either begin in God or in rebellion against him. Systematics affirms the sovereign triune-God; hence, the denial of systematics is the denial of God.
Fifth, it is necessary if we are to think logically and holistically, holding to an ordered, rational, and understandable universe. It declares a meaningful connection between all facts as created by a sovereign God who orders all things according to his own nature and decrees. It provides the basis for preaching and teaching, as did Paul, the whole counsel of God. The alternative is a piecemeal presentation of brute facts where the part – usually one’s favoured perspective – risks becoming the whole.
The main heads of a systematic theology are: God (theology proper), Man (anthropology), Salvation (soteriology), Church (ecclesiology), and Final Things (eschatology).
R. J. Rushdoony, Systematic Theology, Vol 1, (Vallecito, California: Ross House Books, 1994, pp. 59-61).