What Is Faith?

I am taking this almost entirely from Hebrews, Chapter 11.  I won't quote the whole passage because my readers can see it in their own Bibles as we go along.  But I will comment on specific words and phrases from the text.
First a little linguistic background:  The word pistis in Greek is the one translated as faith in our Bibles.  In Koine, the Greek of the New Testament, the word has a much greater area of meaning than our English word faith.  In Greek, as in the Hebrew word rendered pistis in the Septuagent, the word includes an element of obedience and action, not mere mental ascent. Therefore, when we see the word rendered faith in the Bible we must bear in mind the expectation of the writer that it includes the meanings from the original tongues.
The very first verse in Hebrews 11 expands the meaning of faith beyond the Greek or Hebrew concepts in common usage of the day.  According to the writer of Hebrews, faith takes on a concrete aspect in proof of the abstract.  This is important because we overcome doubts produced by circumstances through our firm adherence to that which we believe.  I'm not advocating blind faith in the divinity of, say, a Frisbie or a coffee cup.  Faith must be firmly grounded in the revelation of God in His Son Jesus Christ, and handed down to us in the writings of His followers.
Faith is the weapon by which the spirits of darkness are defeated in our spiritual warfare.  Faith is the tool of the believer to maintain psychological equilibrium in stressful situations.  Faith is the shield by which the attacks of these spirits of darkness are deflected or quenched.  Faith is powerful, but only so because the object of that faith is powerful.
Holding to faith can and will enable the spiritual warrior to obtain victory in the battle over the mind and in the daily struggle of life.  While the preachers of Prosperity go too far into heresy in their abuse of faith, their original premise is correct in that our need and some of our wants are being met by God so long as we allow Him to work through our faith. 
Abraham believed God, and in spite of his many recorded shortcomings, that faith was accounted to him a righteousness.  Enoch walked with God, an application of faith, and then no one could find him because God translated him.  Noah took God at his word and, in a world that had never known rain, built an ark, a huge boat as large as an aircraft carrier, miles away from any water, that God used to save his family and the animals.
So faith is more than something you have in your mind.  Faith is what you do.  I urge my readers to research faith more on their own, then go out and do it.