What is the basis of philosophical justification? Is it reason? Is it practicality? Is it authority?
This is the first problem when trying to rebuild the faith. Some will say Reason is our starting point - heck, I used to say that.
But let's think about that idea in a historical-genealogical way. The ancient Greeks stated that reason was the mark of the gods in man, and that if we shared anything it was reason. Aquinas and other scholastics say that reason is the image of God in man, and that man is essentially a rational animal. But what if we disagree? Is there a reason to attribute reason a special place? or is this just arbitrary.
Using reason we can argue for the necessary being, God. But then what. Can we say that we really understand God? Wouldn't we be saying that we - finite beings - could comprehend an infinite being? (and thus make him finite).
The truth of the matter - I think - is that while we can say 'some necessary being' was required to start this whole thing, you can't say much more than that.
To say that a being would need to give us revelation is irrational. The deists quite easily argued that for God to love or be involved with humanity is not logical, it is gracious, love has no reason, and yet "love alone is credible" (Hans Urs Von Balthasar).
Thomas Aquinas in good patristic tradition said that the only thing we could actually say about God was - what Pelikan called 'the metaphysics of Exodus' - that God is who he is (Ex. 3:14). God is, and so we are. So then the smartest question would then be to ask: what does God say about humanity, reason, revelation, etc. And how can we judge between revelations?
Why should I trust the Bible and not the Qu'ran? In the end, really, there is no argument for why the Christian revelation really is true, except the conviction of the Holy Spirit. This is where I find Rome to be no better than Wittenberg. The Protestant says "God's revelation is true because God said so, and there is no authority higher than God", the other says "God's revelation is true because we say so, and there is no authority higher than us (the Church)".
So what is the Christian supposed to do. By the common grace of reason, he knows some kind of Being must exist. Then we have the Revelation to Israel in the Old Testament, and from that community Jesus of Nazareth, and the apostles of the church. The problem is that reason is uncapable of giving me an answer to tons of issues. Why did God have to become a man? Ockham argued that God could've become a Donkey and saved us. How can we say our finite human reason is capable of comprehending an infinite being. Likewise, even if we just accept the Old Testament, God does some crazily irrational things. He orders Moses to kill his son against the natural law (unreasonable) as old Soren reminds us (Fear and Trembling), and he orders the slaughter of every man, woman, and child of foreign tribes purely by his will, he says he "will have compassion on whom he will have compassion", and when Moses asked to see God's glory, God showed him his backside (Luther implies Butt). None of these are rational things.
I can explain after the fact that Jesus had to be a God-man, as Anselm did, and that makes sense, how God saved us seems rational. Why he saved us, is entirely another matter.
So the Christian is left with reason which has already been shown an inadequate tool at arriving at an understanding of this irrational God. Hans Urs Von Balthasar (I believe paraphrasing Karl Barth) said that God's revelation is like an unfinished symphony of Mozart's. There is no way to 'reason out' an ending to it, it is purely gracious and benevolent, it has no reason to exist, but it does, and yet people's hearts are moved by it.
The grace of God is like music, it is a gift, and gifts are not rational, or rather, they are not for our fallen minds. Scripture and Augustine teach us that humanity is fallen and that their minds have been corrupted by sin. "What has Athens to do with Jerusalem?" asks Tertullian (with an implied answer of Nothing). And the Scriptures warn against vain philosophy, and Paul writes that the foolishness of God is wiser than the wisdom of man (1 Cor 1:25).
Thus no such analogia entis, Analogy of Being, is possible for the Christian. Catholic Thomism proposed such an understanding, but really -as Barth claims- only an Analogy of Faith is possible, because if God is the ultimate truth of the universe, and he has given us his eternal word, and the deposit of faith once and for all (Jude 3), and reason is uncapable of 'explaining' him, then we can only use the faith. The Theology of the Cross is foolishness to the world of rationalism, which makes it's own Theology of Glory. But God is known by the Theology of the Cross, he gives life and takes life, his gratuitous and arbitrary grace and blessings flow where they will, and he descends to a stable to be born, dies on a cross between two criminals, and raises from the dead. There is nothing in Plato or Aristotle that will tell you that is logical, it is a theology of foolishness, a theology of the cross, not a theology of glory.
So for the Christian, we have our only epistemological foundation being God and what he reveals and enlightens via the Holy Spirit, who is "the light which enlightens everyone" (Jn 1:9).
As to which books fall within the Canon, which councils are correct, and which fathers orthodox, I'll give you all of them. The next post will be on Authority for the Christian.
I will be defending the Book of Concord's formulation that doctrine must be:
"supported by firm testimonies of Scripture, and to be approved by the ancient and accepted symbols... [which] ... have... [been] ... constantly judged to be the only and perpetual consensus of the truly believing Church, which was formerly defended against manifold heresies and errors, and is now repeated."
And defend Scripture first, read within the ancient creeds, and in accordance with the first councils, and with the insights of the Fathers, Tradition, and a very careful and limitted use of reason.