The regula fidei presupposes that from the outset the content of Christian faith constituted a unity. The doctrinal system of Christian theology was created neither by a retrospective systematizing nor by the exigency of the fight against heresy, even though the various explanations of this system are secondary. Rather, the content of faith existed originally as a totality, a coherent “system”/“arrangement” (Ordnung), which coincides with the regula fidei and with the divine ordo salutis (Heilsordnung), as witnessed to in scripture. At the same time, this unity is a unity of both testaments. The new covenant or the Gospel, witnessed to by the New Testament, is inseparably connected to the Old Testament, i.e. with the divine reality witnessed to in the Old Testament scriptures. If one were to accept only the gospels or only the New Testament as the basis for Christian doctrine, then the rule of faith would never be heard. Doctrine would be deformed, so that it would not be a faithful representation of the actual way of salvation. The message of the New Testament, in other words, must be interpreted and understood in accordance with (in Übereinstimmung mit) the Old Testament revelation. The content of the Old Testament does not only form the historical background of the Christian religion. Rather, it belongs itself to the same divine plan of salvation, which finds its consummation and conclusive clarification in Jesus Christ. That means that not only is a christological use of the Old Testament from the perspective of the New Testament authoritative. An interpretation of the New Testament from the perspective of the Old Testament must also be authoritative. It is not the case that the antetypes and promises are only to be interpreted from the perspective of the revealed reality and the fulfilment. The witness to the facts underlying the New Covenant (i.e. the deeds and passion of Christ) must also be so understood in such a way that it harmonises with the Old Testament revelation. Together, these two witnesses form a single, unitary order of salvation (Ordnung der Heilsgeschichte).
It is therefore not the task of dogmatics to discover the unity of theology within a basic “Christian idea”(Grundidee des Christentums). Rather, it should assume a literal, primal unity of revelation which is rooted in the actual order of salvation history (Ordnung der Heilsgeschichte), as opposed to retrospectively accomplishing an inner connection between various dogmatic statements. This unity is not the creation of a systematic meditation on the objects of faith, it is much more the actual presupposition of Christian theology in the first place. Even the formal order (äusserliche Ordnung) which exists in the various theological methods can impede a correct presentation of belief precisely because such an order represents an attempt to set up a secondary unity based on a logical or anthropological principle. One thinks, for example, of the res-et-signa method of the Middle Ages, the analytical method of Lutheran orthodoxy or the ordo salutis of pietistic theology. Just as the various idealistic theological systems often signify a genuine deformation of the primal unity (ursprünglich Einheit), so these older methods regularly—even though in themselves pedagogically enlightening—bring about a certain obscuring of the actual “order” (Ordnung), i.e. the inner structure of Christian doctrine.
There is, as it were, a “natural” order for dogmatics, which is not grounded in a common idea but rather in the unity of the Old and New Testaments, an order which, according to its nature, is not so much systematic as historical. It is not only a formal scheme for the presentation of theological teaching, it is rather totally determined by its content. For this reason it coincides with the regula fidei, and the regula is at the same time a witness to this original order, grounded in the genuine events of salvation history.