There is unquestionably an element of understanding to faith. But there is more to it than that. For Luther, Faith is fundamentally trust. He uses the word fiducia, which means confidence. Faith is about trusting a God who makes promises, and whose promises may be relied upon.
Luther wrote in The Babylonian Captivity of the Church, “Where there is the word of God, who makes promises, there must necessarily be the faith of the person who accepts those promises.”
Faith lays hold of promise. Assurance is not established on reason or science but on the apprehension and acceptance of the word of God.
Witsius comments on Hebrews 11:1 by stating that there is ‘substance,’ or hypostasisor existence to the objects of our faith, “the properties and circumstances of things have a hypostatis, that is, really exist, and are not mere figments of our imagination. Accordingly, faith causes the thing hoped for, though not yet actually existing, to exist in the mind of the believer; who assents as firmly to the promises of God, as if he saw the blessings promised already present.”
Calvin also used this term hypostasis when referring to the object of our faith. Calvin states, “Faith is the hypostasis, that the support or possession, on which we fix our foot.”
Witsius also states that, “We understand by the term [faith], a principle which pervades all the faculties of the soul, and is the proper mean of uniting them to Christ, and of thus quickening, and making them holy, and happy.” 
The final resurrection of all men has not yet occurred, nor does it exist in itself, but faith gives it substance in our mind, because we believe God’s promise. The object of our faith; God’s promise of the resurrection, becomes a fact, a historical event just like the battle of Gettysburg.
Likewise, this principle works backward in time to lay hold of the promises of past events. Christ’s declaration in John 19:30 that “It is finished,” though stated in the past and fulfilled in the future, has substance or existence as truth in the present by faith.
Also, in communion we believe that Christ is present in the elements because He said He is, and faith in those words makes the communion, not a figment of our imagination, but something substantive and real. We believe and by believing we come to know that it is not merely bread and wine we hold. And this faith animates our spiritual life and relationship with Christ.
Faith lays hold of God’s words, objects both past and future, and makes them present. This supports the soul, upon which it steadfastly fixes its foot and stands firm.
The Apostle’s Creed, Vol. 1. Witsius, Herman. 43
The Apostle’s Creed, Vol. 1. Witsius, Herman. 44.
The Apostle’s Creed, Vol. 1. Witsius, Herman. 35.