At the risk of greatly oversimplifying things, I believe these two quotes consist of the main disagreement between Protestants and Catholics:
"...unless you understand first of all what your position is before God, and the judgment which he passes upon you, you have no foundation on which your salvation can be laid, or on which piety towards God can be reared" (John Calvin, Institutes of the Christian Religion, Book 3, Chapter 11.1, A.D. 1559).
(yes I stole that from Jay's blog)
"Anyone who is to find Christ must first find the church. How could anyone know where Christ is and what faith is in him unless he knew where his believers are?" - Martin Luther
Which is more important? The Existential (what must I do to be saved? - the question asked of Our Lord) or the Systemic (how can I know unless someone shows me - Ethiopian Eunuch). How does one choose between Aristotle and Kierkegaard?