Most of what dominates conservative evangelical Reformed theology these days is rooted in the speculative tradition, or what, more medievally is known as the via negativa (‘negative way’). Katherine Sonderegger, as a contemporary thinker, typifies it this way. Here she writes:
Note what I said here! Our reading of the priestly-prophetic visio Dei is not principally Trinitarian in character. We are not hearing and seeking out in this witness of ancient Israel a sign and foretaste of Triune Persons. It is not the Father, say, that we see breaking through the cloud and smoke to descend upon Moses and upon the people Israel. Nor do we look for an intimation of the Son in these royal Appearances in the temple. We do not bring forward first and principally the Holy Spirit as personal disclosure in Dame Wisdom or in the maternal brooding over the dark sea at creation’s dawn. The forward press of so much modern theology—the drive to subsume the doctrine of God within the Trinity and the Triune Persons—does not, I believe, properly attest the Unicity of the God of Israel. The Deity and Nature of God is personal: the One God is a Person; we can dare to put it this way. Monotheism is no shame word! At once God is Nature and Person, and the witness of ancient Israel to its Lord is to an Object inalienably Subject, a Subject lowered and handed over to be Object. This oscillation in Israel’s and therefore our religious life before God—now our experience of the I AM—is the gracious condescension of the Lord God to usward, for these are not two, not distinct or segmented, but One, One Mystery, One God.
There is an ancient pedigree to this approach, one laid down by none other than Peter Lombard in the structuring of his Sentences; we might even call this the salvation-history approach. But inherent to this, as illustrated by Sonderegger, is a need to decentralize the threeness of God over-against the oneness; just because this is the ‘order’ we ostensibly receive in the linear unfolding of the Old Testament disclosure. So, in this sense, in the tradition Sonderegger forays forth for us, we could say she is following the contours of Scripture; but we could also say that she is doing so in an abstract way. Her approach can be characterized as ‘abstract’ at the point that she does not principially ground her ‘Word-based’ theologizing in the Christian understanding of the Logos of God. Contrariwise, what we see the New Testament authors doing, the Gospel of John comes quickly to mind, is a retroactive or recapitulatory reading of the Old Testament wherein the Word of God, who is the Christ, seemingly breaks in and all over the Old Testament and sees Christ as the revelation of God all along. In other words, in light of Christ, we come to recognize, by the Spirit, that the Old Testament was a witness to Jesus the Messiah all along. If this is the case, the so called ‘unicity’ of God is never an abstract oneness, but one that is inextricably understood in the multiplicity of the Triune Life of God as revealed in the Son, who is Jesus Christ.
In contrast to Sonderegger, and the tradition she typifies—which is often considered The Tradition of the Church when it comes to theological endeavor in the Western iteration—I want to suggest that we follow the New Testament authors, and understand that our theological entrée must be grounded in the Word of God alone. What I want to introduce us to is not without controversy though. But, I think we can constructively appropriate ways of thinking from contexts that might not end up correlating with the way we end up recasting them vis-à-vis their original context. Here, Eberhard Jüngel, through the telling of David Congdon, helps typify the sort of Word-based theology I think is more principially grounded in the concrete reality of God for us in Christ. Read with me for a moment, and then we will attempt to provide the constructive appropriation I am referring to. Congdon writes:
Jüngel begins by summarizing the way Bultmann differentiates the object of theology from the fides qua creditur of liberal theology, the fides quae creditur of Protestant orthodoxy, and the unknowable God of mysticism. Each of these approaches in theology either loses the divine object of theology altogether or speaks of God in abstraction from God’s “saving deed” (Heilstat) in Christ. If it is not to be mere speculation, theology can be the science of God only as the science of God’s word, the kerygma, the fides quae creditur. But the kerygma is the concrete event in which God’s saving action takes place: it is the eschatological word of God’s justifying judgment in Christ. One can only speak about this event by participating in it and existing as the object of this divine judgment. That is why “theology is the science of God, in that it is the science of faith, and vice versa.” Or as Luther famously put it in his Large Catechism, “these two things belong together, faith and God.” Theology, according to Bultmann, is a particular understanding of God that arises from the event of faith in the word of God. It is a task “enjoined to faith from faith and for faith,” and thus it does not derive from any general account of science or any human capacity for revelation. The encounter with Christ in the kerygma is the sole basis for theological speech and must occur ever anew. According to Jüngel, “the kerygma that faith accepts thus elicits along with faith a cognition [Erkennen], which is a knowledge [Wissen] that is never separable from the event of the kerygma and from the event of faith, but remains related to the Lord who encounters us in the kerygma.” Put more succinctly: “the truth of faith is the event of truth.”
Sounds “existentialist,” right? The emphasis is indeed on encounter, but not an encounter generated by the “I” instead one that comes from the “Thou,” from God in Christ. Some might be concerned that Bultmann’s breath is too close to this to be of value for the conservative evangelical theology. But we can avoid going all the way with Bultmann, and instead critically appropriate the good in the bad. And what we are really being presented with, through Congdon, is Jüngel’s Word-based basis for doing theology that is shaped by the concrete kataphatic reality of God in Christ.
Maybe you also noticed as you read Congdon’s development, the priority that is given to God confronting us, and giving us capacity that we did not have prior to the justification and reconcilation He brings for us in Christ. This, in itself levels a resounding no to the sort of theological method that Sonderegger, and her tradition, gives us. It says no to the inherent analogy of being and natural theology that allows the theologian to speculate about God in the first place; even if that speculation is said to be driven by the salvation-history unfolded in the Old Testament disclosure. For a genuinely Word-based theology there is no space for speculation about who God is, or what God is just because God’s revelation for us is given without remainder in Christ; and our knowledge of God in this frame is fully contingent upon this ‘without-remainder’ givenness for us in the face of God in Christ.
To summarize: Sonderegger, and the tradition she typifies, is committed to a speculative mode for doing theology; a mode that presupposes upon an inherent capacity within the human agent to discern or discover God simply by reflecting on the ‘nature’ of things as they are disclosed in the fabric of the created order. It is this mode of theologizing that necessarily starts with an emphasis on the oneness or monadic quiddity or whatness of God precisely because it starts by speculating about the singular power that might have been able to ‘cause’ what the thinker is discovering via the negative of the negating process they are enthralled by. Contrariwise, the Word-based tradition that Jüngel can help us understand is one that is based necessarily upon the premise that ‘reconciliation is revelation.’ What comes with this axiom is the notion that human agents have no capacity in themselves to discover God no matter how hard they try. In this Word-based approach, the theologian is fully dependent upon God encountering them, rather than vice-versa. Herein, in the encounter, the theologian becomes capacious to know God, but only as God is made known to them, without remainder, in the face of Christ.
I commend the Word-based approach to you; even if we might have to do some constructive work in order to keep it genuinely in line with orthodox premises. The fear of existential theology is unfounded just as it is possible to critically appropriate themes from existentialism, as it developed in the modern period and retext them under the pressures presented by the encounter of God Hisself. The fear of existential theology is unfounded just as the object of our theology is in fact also the Subject of theology; in other words, just as theology is grounded in the personal and Triune givenness of God for us in the person of the Son, Jesus Christ. This, I contend, is the better way forward for doing a genuinely Christian and evangelical theology. A theology that elides speculation about God from our own resources, and instead trusts the God revealed to have the capacity to explain Himself to us as both One and Three, Three in One in the tremendous mystery of the God who is for us, with us, and not against us.
 Katherine Sonderegger, Systematic Theology: Volume 1, The Doctrine of God (Minneapolis: Fortress Press, 2015), Loc. 6577, 6584 Kindle.
 David W. Congdon, The Mission of Demythologizing: Rudolf Bultmann’s Dialectical Theology (Minneapolis: Fortress Press, 2015), 472-73.
 The bad being Bultmann’s understanding of the bodily resurrection of Christ and its untetheredness from historical concrete reality. We can follow Barth’s understanding of resurrection while at the same time appropriating some of Jüngel’s Bultmannian-like approach to grounding theological approach in the concrete reality of the Word of God in Christ. Here is Barth’s understanding of resurrection, just for point of reference:
The Easter story is not for nothing the story whose most illuminating moment according to the account of Mark’s Gospel consists in the inconceivable fact of an empty sepulcher, a fact which (in producing atrembling and astonishment) lays hold of the three woman disciples and reduces them to complete silence for they told no one of it, for they were afraid (Mk. 16.8). Everything else related by this story can be heard and believed in the very literalness in which it stands, but can really only be believed, because it drops out of all categories and so out of all conceivability. It cannot be sufficiently observed that in the most artless possible way all the New Testament Easter narratives fail to supply the very thing most eagerly expected in the interests of clearness, namely an account of the resurrection itself. CD I/2 §14, 115