Love this quote about faith from George Foot Moore's book, History of Religions, Volume 2, pg. 127, PDF pg. 145, (Moore was a Professor of the History of Religion at Harvard University, and his book Judaism is considered the book on Judaism in the time of Jesus.):
"Paul's gospel is the gospel of the cross; the death and resurrection of Christ are the foundation of the Christian religion. Without them there is no salvation for Jew or gentile; in them there is salvation for all mankind. Christ's death is potentially the death of all; in his resurrection humanity triumphs over death and attains eternal life. But only those are actually saved who are so united with him that they were crucified with him and with him are risen from the dead.
This union is effected by faith, which is not merely the intellectual and moral acceptance of the gospel, not simply a trust in God or in Christ, but an undefinable experience in which oneness with Christ is permanently realised. This is the characteristic Pauline conception of faith, the most distinctive feature of his doctrine of salvation. It has no antecedent or analogy in Judaism or among Jewish believers in Jesus, and none in the personal religions (mysteries) or philosophies of the time; it sprang from Paul's individual experience–an experience peculiar both in its outward circumstance and in the temperament of the man. For this reason his mystical idea of faith made little impression on early Christian thought, which held fast to the ordinary and accepted meanings of the word, belief and trust.…"
I'd put it this way, faith is walking in the spirit. That's the mystical part Moore is trying to explain.
In order to walk in the spirit you must see in the spirit, you must see with your spiritual eyes. You can no more use your physical eyes to walk in the spirit than you can use your physical eyes to smell roses. Your physical eyes are as blind to the spirit world as they are blind to the scent of roses.
You can no more use your reason to see with your spiritual eyes than you can use your reason to see with your physical eyes. That's why the most ignorant, uneducated, uncivilized, person in the world can walk by faith; while the greatest philosophical, scientific genius misses it. It's not about what you can figure out, it's about what you can see and if you purpose to keep your eyes shut you're guaranteed to not see anything.
We call it "faith" because it deals with things that are invisible to our natural eyes. If we walked only in the spirit we wouldn't call it faith we'd call it sight. But we are amphibians, as C.S. Lewis said, sometimes moving in the spirit world, mostly moving in the physical world. So when we walk by faith, when we walk in the spirit, when we perceive something with our spirit, we distinguish that perception, from what we usually perceive when walking in the material world, by calling it faith. From the perspective of our senses it is faith, but from the point of view of our spirit it is something we know to be true, it's something we have perceived. I don't "believe" Jesus Christ rose from the dead because I've examined the facts and determined that it is reasonable to conclude that he rose from the dead. I "believe" he rose from the dead because I've perceived that with my spirit.
Sometimes you'll hear evangelicals saying, ' Well faith is just trust!' Then just trust me, trust isn't just trust. Trust is based on perception. Once you perceive something your response is natural. You perceive that your house is on fire, you don't respond by saying, "I trust the house is on fire, I trust the house is on fire, I trust the house is on fire." Once you perceive the house is on fire your response is natural… "Lets get out."
Once you perceive that Jesus has been exalted to the right hand of God, been given all authority in heaven and in earth, that he has been made true King over all the earth, once you perceive this, your response to it is quite natural, "Jesus is Lord!"