I just wanted to pipe up and say that it's been quite interesting reading everyone's interpretation on "faith"! Yes, I left it mostly as an exercise to the reader rather than imposing a single "definition" to the term... We had a pretty good discussion about this topic on Aikido-L so I thought I'd bring it over here.
For me personally, faith is a very important part of my aikido training. Although it's not religious in nature for me, it's more philosophical and/or spiritual.
Many people here have talked about "proof" and such being contrary to "faith." For me, I've felt many things in aikido from a lot of experienced people that I personally can not reproduce myself. Yet, I keep training in the hopes that I will, some day, through training in a manner laid down by these people, be able to do at least some of them. That, to me, is a sense of "faith" in my training. To this date, I may not have any proof that I personally will be able to do something, but I keep training.
Lastly, I wrote the following on Aikido-L so I'll just copy and paste the whole paragraph:
I believe one of the principles that Ki Society folks use is (in English), "Perform with confidence." Putting aside the semantics of the word "perform," I like the word "confidence" there. From what I understand, its etymology comes from confidere, from com- + fidere (to trust, have faith in). This sense of moving with trust/faith in oneself is important, I think, after a while. I don't see many shihan out there who are questioning their own abilities, after all.
Thanks to everyone for an interesting discussion (so far)!