My father said in a comment that he was still wondering how we knew that God is a God of reason. I thought I remembered a couple of reasons, so I thought I'd go back and look for them:
1. In the beginning of John, the word we normally translate for "Word" is "Logos", which also means "reason". This is not the first time that I've heard that translating that as Word strips away some of its meaning. When Jesus is a concept which reflects both "reason" and "word", and He is one with God, we know that those things are important to God's nature. Language, with which we structure concepts not only for communication, but also for our own thinking. Reason, thinking in ways that fit together and make sense, perhaps? I find Reason much harder to define than Language. Anyone have any ideas about that? Regardless, Language and Reason are similar: they both bring order and structure to thought. Perhaps that reflects some of the meaning of Logos. In the beginning was the Logos, and the Logos was with God, and the Logos was God.
2. The Pope refers to some other church writers, in particular Lateran. No idea what Lateran says.
3. In general throughout the talk, the Pope refers to this union of reason, represented by the Greek spirit, with faith, as something that came about in the very dawning of the church. He seems to be saying, "You may not be able to find a great deal about this in scripture, but it has been with us from the beginning, and that is not an accident. Christianity belongs with that Greek tradition of reason." Not so much an attempt to prove, I don't think, as an appeal to the roots of the faith. Not the same as an appeal to tradition, as tradition tends to accrete and build up as the centuries go by. Rather, an appeal to foundations, to the original form of the faith. I think that this makes it worthy of serious consideration, but it is nothing like finding it in Scripture. I have always thought that God was rational, but I don't know how I was taught it. I do recall that his foolishness is greater than our wisdom... Rationality is easy to accept, but I'm not sure what value I'd assign to Greek culture and its links to us.
4. The New Testament was written in Greek. This serves as an example of how tightly bound Christianity and Greek were at the time the scriptures were written. Perhaps there is something to be learned about God here, that He brought things to such a pass that the New Testament was not written in Hebrew, not written in Latin, but in Greek. I'm not sure. I think it's more about the tightness with which the roots of Christianity hang to Grecian soil.
I do not thing that the points I have given above are enough to establish the rationality of God on their own, nor enough to establish the importance of the link between Christianity and Greek thought. I'd appreciate more thoughts on this, in either direction.