Faith Is A Verb
The idea of faith is so central to understanding the Bible that I think it's worth talking about over and over. I hear people referring to faith as a mental state with the connotation of having doubt. That's typical of the modern use of the word faith, but really isn't the way it's used in the NT.
Webster's dictionary gives several different definitions for faith. The first is "unquestioning belief that does not require proof." Another definition is "a religion or system of religious beliefs." Those are the ways that most people today think of and use the word "faith" - as something believed without proof and/or a religion. Webster's also has the definition, "complete trust, confidence, or reliance." This last definition is closer to the way "faith" is used in the Bible, but even that definition is incomplete.
The problem we have in translating the Bible is that sometimes there is no single word in English that can represent the Greek word. That's the situation here. When you see words like belief, believed, and faith in the NT, they are really the same word in the Greek although they may have slightly different meanings due to context. That word is "pistis" as a noun and "pisteuo" as a verb. When the noun form is used it is translated as faith or belief. When the verb form is used it is translated as believed, or as "by faith."
The ISBE entry gives a good, brief overview of the word "faith".
The history of the English word is rather interesting than important; use and contexts, alike for it and its Hebrew and Greek parallels, are the surest guides to meaning. But we may note that it occurs in the form "feyth," in Havelok the Dane (13th century); that it is akin to fides and this again to the Sanskrit root bhidh, "to unite," "to bind." It is worth while to recall this primeval suggestion of the spiritual work of faith, as that which, on man's side, unites him to God for salvation.
Studying the word "faith" in the light of use and contexts, we find a bifurcation of significance in the Bible. We may distinguish the two senses as the passive and the active; on the one side, "fidelity," "trustworthiness"; and "faith," "trust," on the other. In Gal_5:22, e.g. context makes it clear that "fidelity" is in view, as a quality congruous with the associated graces. (the Revised Version (British and American) accordingly renders pistis there by "faithfulness.") Again, Rom_3:3 the King James Version, "the faith of God," by the nature of the case, means His fidelity to promise. But in the overwhelming majority of cases, "faith," as rendering pistis, means "reliance," "trust." To illustrate would be to quote many scores of passages. It may be enough here to call attention to the recorded use of the word by our Lord. Of about twenty passages in the Gospels where pistis occurs as coming from His lips, only one (Mat_23:23) presents it in the apparent sense of "fidelity." All the others conspicuously demand the sense of "reliance," "trust." The same is true of the apostolic writings. In them, with rarest exceptions, the words "reliance," "trust," precisely fit the context as alternatives to "faith."
Another line of meaning is traceable in a very few passages, where pistis, "faith," appears in the sense of "creed," the truth, or body of truth, which is trusted, or which justifies trust. The most important of such places is the paragraph Jam_2:14-26, where an apparent contradiction to some great Pauline dicta perplexes many readers. The riddle is solved by observing that the writer uses "faith" in the sense of creed, orthodox "belief." This is clear from Jam_2:19, where the "faith." in question is illustrated: "Thou believest that God is one." This is the credal confession of the orthodox Jew (the shema?; see Deu_6:4), taken as a passport to salvation. Briefly, James presses the futility of creed without life, Paul the necessity of reliance in order to receive "life and peace."
A few detached remarks may be added: (a) The history of the use of the Greek pistis is instructive. In the Septuagint it normally, if not always, bears the "passive" sense "fidelity," "good faith," while in classical Greek it not rarely bears the active sense, "trust." In the koine, the type of Greek universally common at the Christian era, it seems to have adopted the active meaning as the ruling one only just in time, so to speak, to provide it for the utterance of Him whose supreme message was "reliance," and who passed that message on to His apostles. Through their lips and pens "faith," in that sense, became the supreme watchword of Christianity.
I've emphasized that one phrase in the last paragraph to show something important in understanding the difference between the modern idea of "faith" and the NT use of "pistis". Today we think of faith as something passive. To the NT writers, faith was an action. As some of us like to say, "Faith is a verb." If you don't understand faith in that sense, you cannot understand what the gospel and epistle writers are saying.
Another important thing to note here is that "faith" includes in it the core meaning from the Indo-European root, "bhidh". That is the source of our words "bide" and "abide" and it carries the connotation of something firm, and of a binding together of two things. The Latin "fide" the English "faith" and the Greek "pistis" all have this same origin. To "have faith" is to firmly bind yourself to something. Hence, it also refers to "fidelity" to a covenant relationship.
Faith as used in the NT is the opposite of doubt. For example, in Mat 14:31 Jesus says, " And immediately Jesus stretched forth his hand, and caught him, and said unto him, O thou of little faith, wherefore didst thou doubt?" Doubt translates a Greek word that means literally to "double think" or to divide the mind between different possibilities. In other words, Peter mixed faith and non-faith in his mind. The faith that Jesus calls people to means to have no doubt about something.
Most importantly, faith is an activity. In our modern way of thinking we separate mind and body, subjective and objective. We say that the objective things are provable and subjective things are not and then further claim that only what is objectively provable is "real" and "true." People today put faith in the category of something subjective and thus perceive it as believing without proof. That's a modern idea that shouldn't really be read back into the NT. In many cases, when Jesus healed someone, he would say, "Your faith has made you whole." Their faith was not simply sitting around passively believing that Jesus could heal them. It was a belief so strong that they went and sought out Jesus. That faith-action resulted in the proof or validity of what they believed. Thus, faith in the NT sense is an action based upon a belief. The belief is so secure that the person is willing to act, even to the point of risking death, because of the confidence in what they believe.
In the gospels, the phrases "believe on" and "believe in" are used. These are not exact translations, however. In almost all instances it is a translation of "pistis eis" which literally means "faith into". The preposition "eis" indicates a direction towards and into something; moving from the outside to the inside in other words. The meaning is that you direct your faith towards something, but to say it that way in English sounds strange. It does, however, give a slightly better indication that what the NT writers are indicating is something active not passive.
There are many analogies that can be used to all represent this. One example is when a person goes to the Post Office to mail a letter. Putting the letter in the box and walking away without worry about whether or not the letter will arrive at its destination is what the NT is describing as faith. When the person mails the letter he has a lack of knowledge of how the Post Office will deliver the letter. Nevertheless, he is willing to trust that correspondence to the Post Office because he is convinced they will deliver it as promised. In so doing, he has directed his belief "towards" the Post Office and acted accordingly without doubt in the outcome.
NT faith is the situation where you may lack information as to how something happens, but a complete confidence that it will happen and that confidence leads to specific action. In response to that action we obtain evidence that the trust was not misplaced (See Heb. 11). It is not a confidence in our own understanding and abilities but rather a confidence in God and His promise to us that causes us to act. We trust the Post Office to deliver mail and act accordingly. We trust in God to deliver us through the working of the Holy Spirit and act accordingly.
Faith is a verb, as we say. Or simply, "The just shall live by faith."