There is no small chance that Matthew 16:15-17 contains all that is necessary for a theological epistemology. The knowledge of Jesus that is articulated here is a product of nothing else than God’s own activity, God’s own revealing action, within the context of reconciliation. There are a number of things that can be drawn out here. First, the knowledge of God that is articulated here is a product of grace: God’s own free action to reveal himself. It is only through God’s own action that God is revealed. This first point implies a second point: that if the knowledge of God is had by grace alone, it is a gift. A third point: knowledge of God is knowledge of God, and as such revelation of God is revelation of reconciliation. We can even go a bit further than that and say, with Barth, that revelation is reconciliation. Fourth: if revelation is reconciliation, then necessarily the setting for revelation is the covenant within which God acts towards the world (the covenant is the internal basis of creation and creation is the external basis of the covenant).
The first point, that the knowledge of God is a product of grace, negates any purely human or any independent effort to achieve knowledge of God. This rules out some varieties of natural theology – any attempt to move from knowledge of the world to the knowledge of God. T.F. Torrance noted that Barth’s rejection of natural theology wasn’t natural theology’s
…rational structure but its independent character, i.e. the autonomous rational structure which it develops on ground of ‘nature along’ in abstraction from the active self-disclosure of the living God.’ (The Problem of Natural Theology in the Thought of Karl Barth)
The second point, that knowledge of God by grace is a gift, establishes an important point: the identity of both the gift and the Giver. For both Torrance and Barth, grace was not a thing but a person: Jesus Christ. In giving Jesus, God gives himself.
The third point: revelation and reconciliation are identical:
Revelation in fact does not differ from the person of Jesus Christ nor from the reconciliation accomplished in him. (Barth, CD I.1 p. 119)
Why is this so? Because knowledge of God requires transformation and closeness to God. There has to be fellowship between God and man for God to be known by man, and this occurs in reconciliation (or, as we saw above, occurs in Christ):
God cannot be properly known at a distance. Theological knowing requires that God establish fellowship with us, a relationship that cannot but be not only cognitively illuminating but also personally transformative. We must be given the eyes to see and the ears to hear. For this reason Barth speaks of God’s address as the “transposing of man into the wholly new state.” The knowledge of any object has far-reaching and determinative consequences for the knower. We exist in relationship with, though distinct from, the objects of our knowledge both past and present. Barth provisionally defines knowledge as “the confirmation of human acquaintance with an object whereby its truth becomes a determination of the existence of man who has the knowledge.” If this is so for ordinary objects, then how much more significant (and radically different) must the determination of the existence of the knower be in the human acquaintance with God. (Kevin Diller, Theology’s Epistemological Dilemma, p. 61)
The fourth point: flesh and blood do not reveal God. Revelation is not observed directly precisely because revelation is indirect and, to use Barth’s favourite term, veiled. There is a dialectic of veiling and unveiling in revelation:
There is no direct factual observation of revelation because revelation is indirect and mediated. In the dialectic of veiling and unveiling, even when God unveils he does so by veiling. What we observe is not the revelation but the veil. Moreover, the revelation is actualistic. It is not always there, at our disposal whenever we want to examine it. (D. Paul La Montagne, Barth and Rationality, p. 167)
Thus, revelation is not something simply given. It cannot be read off of (in Matthew’s case) the historical person of Jesus of Nazarareth. What is observed by Peter is the veil, which is dialectically unveiled by grace. Any knowledge of God that isn’t mediated, indirect and veiled/unveiled is ruled out by Matthew.
Fourthly, the context for the preceding three points is the one covenant of grace. In Christ, the covenant is God’s determination of his ways with the world. All of salvation-history occurs within God’s covenant determination towards the world, and if knowledge of God is fundamentally just reconciliation, then the covenant is essential for knowledge of God. Knowledge of God can, therefore, only take place in a covenant context.