David Congdon offers an insightful quote on the distinction between being an existential theologian versus a sapiential one; for his purposes he is using this distinction to help draw some lines between Karl Barth and Rudolf Bultmann. Whoever the characters are, whether Luther/Aquinas (which is the original pairing), Barth/Bultmann et al. I think the distinction is an instructive one, and so I thought I would share it.
Existential theology is the way of doing theology from within the self-actuation of our existence in faith, as we submit to God in the obedience of faith. Its affirmations are so formulated that the actual faith and confession of the speaker are not merely necessary presuppositions but are reflexly thematized. Sapiential theology is the way of doing theology from outside one’s self-actuation in the existence of faith, in the sense that in its doctrinal statements the faith and confession of the speaker is the enduring presupposition, but is not thematic within this theology. This theology strives to mirror and recapitulate God’s own thoughts about the world, men, and history, insofar as God has disclosed them.
The relative distinction is something akin to doing theology from below or from above; the existential (below) would be more soteriologically/theoanthropologically oriented while the sapiential (above) would be more theology proper oriented; while the dialectical, we might constructively surmise, might be located in the Christological frame (where the below and above intersect and implicate in the singular person of Jesus Christ as the Theanthropos).
 Otto Hermann Pesch, “Existential and Sapiential Theology—The Theological Confrontation Between Luther and Thomas Aquinas,” in Catholic Scholars Dialogue with Luther, ed. Jared Wicks (Chicago: Loyola University Press, 1970), 61-81, at 76-77 cited by David W. Congdon, The Mission of Demythologizing: Rudolf Bultmann’s Dialectical Theology (Minneapolis: Fortress Press, 2015), 71-2.