Oddly enough the concept of faith was never full articulated till college. I believed in the dictums of christianity because it was a plausible hypothesis to answer the questions I had at the time. I was "saved" when I was 11 or so, fearful of dying, uncertain about the world and already asking too many questions. Christianity was introduced to me through growing up in a believing family and going to a "class" led by our then pastor. It was an answer, one that I accepted based on the knowledge I had then. Faith was identified as being identical with the system, not as a means of knowledge, which is how it is used by believers. It would seem I was already a lay-scientist and just didn't know it.
With that mindset in place, Christianity was about answers and that meant study. If this was indeed the source, handed down by deity, for the answers to life, love and happiness then I was determined to plumb its depths. What shocked me then and causes me to laugh sadly now is how few believers look at their beliefs that way. Few then and even fewer now know even the rudiments of their belief systems and don't seem to care about having an utter lack of knowledge of a system that supposedly offers the truth about everything. But I consoled myself with an elitist mentality and thoughts of leadership and a growing cynicism that the majority simply couldn't search enough, that "knowing Jesus" was enough for them even if they had hardly a clue what that actually meant.
Then 9/11 happened. At this point I was already teetering. I had become convinced of biological evolution and the attempt to reconcile that with a literal interpretation of the book of Genesis was disconcerting to say the least. Also, I had embarked since 16 on a quest of studying Christian philosophy in an attempt to answer more questions, finally reading some of the "other side" material rather than simply taking it for granted that Christians writing about it would tell the truth. Finding out I had been lied to about other people's beliefs and systems of thought was quite the blow. I hardly look at it as surprising these days as given the lack of knowledge concerning their own system of belief, knowing anything about others or even caring about the value of real rational persuasion is too much for the typical believer.
But back to 9/11. To say it was shattering is an understatement as undoubtedly it was for a great many. But given my mindset then, it became a lightning rod for what faith-based thinking could lead one to do. Yup, I had finally begun fleshing out what "faith" meant. Clearly it couldn't be identical to "reason" otherwise we'd just use "reason," so what exactly is it? Faith is a means of acquiring or delineating knowledge. To say "I have faith" in something is to declare one's intention of trusting in a proposition despite or because of a lack of reasons. I have "faith" that the ceiling above me won't spontaneously cave in because while I have a rudimentary understanding of physics, construction and mechanics, my lack of knowledge leads me to trust that those who do have a better grasp of such things performed their jobs correctly.
However, here the similarity ends when such "faith" is applied to religious propositions. Like Paul's dictum that "I believe so that I may know", faith is not about trust but about blind acceptance. The nature of the beliefs acquired, the non-material entities of god and spirits, means that they have no connection with anything capable of being thought about. You cannot discuss the non-material, the non-physical by any means open to a physical body and brain, the very concepts are devoid of definition as they are simply "what is not material or physical." This lack of an affirming definition leads to a usage of "faith" that is tantamount to willful delusion, it serves as an equally undefined way of knowing something that is impossible to know. It is the leap into not merely the unknown but the unknowable, not because of any limitation to human knowledge, but because there's nothing defined to figure out. "Faith" is offered as a solution, but one the process of which is undefined (there are no rules or regulations for faith, as there are with reason). The result is a complete lack of justifiability, at least in the sense of differentiating systems. The Muslim, the Christian, the Orthodox Jew and for that matter Wiccans, all use "faith" to justify their systems of thought, despite completely contradictory ends. Some may tout their "holy book" but to believe it as holy requires the very same "faith." The result is not rational discourse, not a forum for discussing various competing theorems, but rather willed compliance. It is little wonder that violence is so often used by Muslims and Christians, their basis for knowledge is nothing short of might makes right.
In the end, I could not adhere to a system that no longer answered any questions and those answers offered found to be even remotely legitimate, like morality, could be found through other avenues of pursuit without the baggage of spirits, archaic and vengeful gods and a denial of reason.