First, God’s Word is truth. It is pure truth, revealed by God, and it is the sole and sufficient final arbiter between what’s true and what’s false.
Second, while we may not understand any doctrine exhaustively, we can nonetheless be confident that what we do know accurately is true. That’s the beauty of propositions. They recognize that truth by definition includes facts, and even though no finite set of facts or propositions ever exhausts all truth about God, we can know lots of true facts about God, and we can even know God Himself (albeit through a glass, darkly) because those facts, and God Himself, have been revealed to us by God Himself in Scripture.
So (lo and behold!) I can actually affirm penal substitution without being guilty of “reducing” the gospel to only that one point.
** Also check out the great motivational posters using the emergent lingo. Good stuff. Makes me miss Purgatorio anew.