In brief? Hooo boy... Well, to way oversimplify things, the Western Church "rediscovered" Aristotle in the 11th c (mostly through its renewed encounters with Byzantium, where academic humanism had survived as a separate institution from the church, and from monastery libraries where a few texts had survived, and from contact with the Arabs - who had recieved their texts from the same Byzantine academics).
Aristotle had come to the conclusion that there existed a God - and actually one that was semi-close to the Christian sense of God. He had done so through rationalistic argument (not through direct revelation by God in Christ, as the church had recieved it).
So a theory developed, now called "scholasticism," that theology could and ought to be subjected to rationalist inquiry. Aristotles method of logic (syllogisms) became, for SOME elements in the West (it was hotly disagreed with at the time), the primary means of furthering inquiry into theology.
Theology, in other words, became academic (rather than mystic). This doesn't mean that reason had no place in theology prior to this - but rather that the idea that we could use revelation as a foundation from which to conclude new theologies, or that we could use deduction to justify (as if from a purely agnostic starting point) Christian theology... this was new.
This is the legacy of Anselm of Canterbury (who used deduction, presented in classic-style dialogue, to argue that God became man in order to pay a sufficient sacrifice to cover the penalty of sin in his text "cur deus homo"; he also used logic to attempt to prove God's existence in his ontological proof), and of Thomas Aquinas. Thomas Aquinas was controversial in his day, but came to absoluletly dominate Western theology (especially after Trent), which is why scholasticism is such a difference between East and West.
In theory, I don't have an issue with using reason to meditate on the faith (which was the goal of the scholastics), because they attempted always to adhere to the faith if reason didn't suffice to prove it (they assumed their reasoning was at fault, not the faith). But a few critical things occured as a result that I'm not ok with:
- They sought to use reason to further develop the faith - as if the faith were not a revelation, but a starting point which could stand up to rationalist inquiry and thereby be developed (like any other academic field).
- Once subjected to reason, the question of "which is right if they conflict?" becomes inevitable. The scholastics answered "the faith" and by this remained Catholic. Later humanists would answer "reason" and by this created the "Age of Enlightenment" and the now-almost-complete destruction of Christian culture in Western Europe.
- They began to glorify human reason (as this was the legacy of the humanist pagans they were imitating) - this led to renaissance humanism, which is the root of an awful lot of modern ills.
For our purposes, though, the reason this presents a difference between East and West is because the exact same humanist tradition was, over time, rejected by the Church in the East. Eastern Christians were fine so long as their study of Aristotle or Plato was purely academic (i.e. much in the way we may study Buddhism as a separate set of ideas because encountering new ideas has academic merit).
When humanists began to try and reconcile their philosophy with Orthodoxy (often having to shift or modify Orthodoxy in the process), they were soundly rejected. The most pronounced case of this occured in the 14th century, when the humanist Barlaam was excommunicated (and then became Catholic; a common theme of this time period that, in all acutallity, caused the Renaissance). The Byzantine humanists we rejected became heroes of the early Renaissance in Italy. It isn't like humanism died in one place and arose in the other. It was a direct transfer of succession. Only in the West there was capitulation to it (to a point - the Catholics certainly reject the hardcore rationalism of the late Renaissance).
So what's the alternative if not rational inquiry? Obedience and mysticism. We recieve the faith, and we hand it on - that's obedience to the tradition. And most of our theological works aren't about a philosophy of theology - they are about prayer. Because prayer is how we come to know God. And what we learn there cannot be put into words like a philosophy, because words cannot contain the Word. Rather, we allow for several wordings because of the divine mystery. This is true to a point in Catholicism, but you've seen the catechism - there are many "developed" (rationally theorized) doctrines one must adhere to. There's no way the early church believed in a bank of merits controlled by the pope to be dispensed through indulgences. Yet that is in the catechism because it was deduced, using scholastic inquiry, in the medieval era.
The overemphasis on rationalism in the West has led to a gradual decline of mysticism. This is most obvious in the (even more rationalist, though using the Scriptures as the foundation / starting point) protestant traditions, but even in Catholicism there has been a decline in asceticism. Mystics and theologians, in the west, are seen as two categories. In the east, the mystics ARE the theologians.
And, to us, the mystical approach is the older. St. Irenaeus of Lyons, in the 2nd century, didn't appeal to philosophy to refute the neo-platonic gnostics - he appealed to traditional interpretations of Scripture, as upheld by the successors of the Apostles (the bishops). This traditionalism, along with the monastic (and prior to that, martyr-centered) mysticism, formed the core of the theology of the 1st millenium church. It used reason to defend the faith from heresy, but not as a primary means of inquiry into the faith.
That is way oversimplified... and I painted with too broad a brush. But I hope it is coherent / lucid enough... Forgive me if the brush was too broad.
In Christ,
Macarius