Formally: The scriptures are a witness to God’s self-revelation in Jesus Christ (Karl Barth).
Materially: The scriptures narrate the story of Israel’s God who raised The Israelite from the dead (Robert W. Jenson) and ushered in the kingdom (Pannenberg and NT Wright).
Implications: At this point I am simply arguing based on the Hebrew scriptures alone. We can debate whether this OT canon was formally closed or not, but neither the Sanhedrin of Jesus’s time, nor Jesus himself, seemed too bothered about a “horizon-less” canon.
Corollary: By anchoring my view in the Hebrew Scripture (and seeing the NT documents as supplementary witnesses to the revelation of the Kingdom in Jesus of Nazareth, again Jenson), I am doing an end-run around the claim that I am accepting the documents of the church but rejecting the church. I think that claim is silly, but I will pretend it holds water for the moment.
If one doesn’t anchor sola scriptura in some form of “narratival epistemology” or “narratival ontology,” then its hard to see how it can stand against more tradition-based claims.