Nicholas of Cusa (1401-1464), of whom I have written a biographical sketch elsewhere, wrote a work called De Pace Fidei (On Peace in Religion), which is a dialogue between the adherents of various religions. In the dialogue the following interesting discourse on justification appears:
Thereupon Paul, a teacher of the Gentiles, rose up, and by the authority of the Word spoke the following:
Paul: It is necessary that we show that salvation of the soul is not obtained by works, but rather from faith, for Abraham, the father of the faith of all those who believe, whether Arab, Christian, or Jew, believed in God, and he was considered as being justified. The soul of the just will inherit life everlasting. Once this is admitted, these varieties of ritual will not be a cause of dissension, for as sensible signs of the truth of belief these things that have been instituted and received as signs are capable of change, not so the thing that is signified.
Tartar: Tell us how, then, does faith save?
Paul: If God should promise certain things because of His liberality and generosity, should not He, Who is able to provide all things and Who is truth, be believed?
Tartar: I’ll have to admit that. No one can possibly be deceived who believes Him, and if he fails to believe him he would not be worthy of obtaining any gift.
Paul: What, therefore, justifies him who obtains justice?
Tartar: Not merits, otherwise this would not be something gratuitous, but a debt.
Paul: Very well put, but because no living person can be justifed through works in the sight of God, but only gratuitously, the Omnipotent gives whatsoever He will to whomsoever He will. Then, if anybody would be worthy to acquire a promise that was purely gratuitous, it is necessary that he believe in God. It is in this, therefore, that he is justified, because from this alone will he obtain the promise, because he believess in God and expects the Word of God to take place.
Tartar: After God has promised something it is certainly just that He keeps His promises. The person who believes Him is justifed rather through the promise than through its faith.
Paul: God, who promised the seed of Abraham, in which all were to be blessed, justified Abraham, that he might acquire the promise. But if Abraham had not believed in God he would have obtained neither justification nor the promise.
Tartar: I agree with that.
Paul: The faith, therefore, in Abraham was only this, that the fulfillment of the promises was just, because otherwise it would not have been just, nor fulfilled.
Tartar: What did God promise?
Paul: God promised Abraham that He would give him this one seed in the person of Isaac, in which seed all races would be blessed, and this promise actually took place. Since according to the ordinary laws of nature it was impossible for Sarah, his wife, to conceive or give birth, yet because he beleived he acquired a son, Isaac. Later on God tempted Abraham, in that He asked him to offer and slay the boy Isaac, in whom His promise of the seed had been fulfilled. And Abraham obeyed God, believing no less in the future promise, even though it would involve the resuscitation of his dead son. When God discovered this faith in Abraham, then he was justified, and the promise was fulfilled in this one seed which descended from him through Isaac.
Tartar: What is this seed?
Paul: It is Christ, for all races have obtained in Him a divine blessing.
Tartar: What is this blessing?
Paul: The divine blessing is that final desire for happiness which we call eternal life, about which you have alerady heard.
Tartar: Do you desire, therefore, that God should promise us the blessing of eternal life in Christ?
Paul: That is what I wish. For if you believe in this same way you will be justified along with the faithful Abraham, and obtain the promise that was found in the seed of Abraham, Christ Jesus, and that promise is the divine blessing.
Tartar: Do you mean to say, therefore, that this faith alone justifies and enables us to attain of eternal life?
Paul: I do. - from “De Pace Fide,” in Unity and Reform: Selected Writings of Nicholas De Cusa, ed. John P. Dolan (University of Notre Dame Press, 1962), pp. 227-229
Note that this was written in 1453, thirty years before the birth of Martin Luther.