"...But that faith which justifies is not merely a knowledge of history, [not merely this, that I know the stories of Christ's birth, suffering, etc. (that even the devils know,)] but it is to assent to the promise of God, in which, for Christ's sake, the remission of sins and justification are freely offered. [It is the certainty or the certain trust in the heart, when, with my whole heart, I regard the promises of God as certain and true, through which there are offered me, without my merit, the forgiveness of sins, grace, and all salvation, through Christ the Mediator.] And that no one may suppose that it is mere knowledge, we will add further: it is to wish and to receive the offered promise of the remission of sins and of justification."
The Book of Concord - Concordia Triglotta Edition, "Apology of the Confession: Of Justification"; (Kindle Edition).