A few months back I posted a two-part review of The Real Luther by Franz Posset (Part one, Part two). Posset is a Roman Catholic Luther scholar. It is Posset's conclusion that "the historical Luther's doctrine of justification is identical with the one of Saint Bernard" (p.127). His argumentation spans many pages. Here's a brief overview. If you're interested, I suggest buying the book. This may be one of the most interesting Luther books I've read in years.
Posset argues that in the Augustinian friary Luther became acquainted the writings of Bernard of Clairvaux, particularly how Bernard interpreted Paul (p. 93). Posset begins with Melanchthon's memoirs of Luther:
And [Luther] told that he was often encouraged by the conversations of a certain old man in the Augustinian College at Erphord, to whom when he set forth his worries. He heard the old man discuss much about faith, and he said that he was lead to the Creed, in which it is said, "I believe in the forgiveness of sinners." That old man had interpreted this Article so that it should be believed not only in general, i.e. forgiven by some persons or others, as they believe Demons are forgiven by David or Peter, but that is was a commandment of God that we believe that the sins of individual men are forgiven by us. And he said that this interpretation was confirmed by a saying of Bernard, and then he pointed to a place in his sermon on the Annunciation, where there are these words, But in addition that you might believe also this, that sins are given TO YOU individually, this is the testimony, which the Holy Spirit bestows in your heart, saying, Your sins are forgiven by you. For the Apostle thinks thus, that man is gratuitously justified through faith. [source]
The capitalization of "TO YOU" of the Bernard citation was actually placed there by Melanchthon. Posset notes,
Melanchthon reported that Luther told him that this quotation was Pauline teaching as it is incorporated within Saint Bernard's sermon: "For thus the apostle concludes that 'a person is justified gratis by faith." This is a contraction of Rom. 3:24 and 28. Thus it was the young Luther at Erfurt who was told that this is Saint Bernard's teaching. And, as such this teaching would become Luther's center piece of his "reception of Paul" as it is the central locus of the Apostle's entire Letter to the Romans and as Luther would say later it is the central theological point of all of Scripture. (p. 98)
Posset identifies the Bernard writings in question as Bernard's Sermons on the Annunciation, with an emphasis on Sermo in annunitiatione domine "Saint Bernard's First Sermon on the Annunciation" (Ann 1). He states, "Ann 1 is the one text from Bernard's biblical theology that led Luther to what we customarily call his Reformational insight of salvation by grace alone through faith alone" (p. 104). Luther went on to either quote or hint at Bernard more than 500 times (p.94). Posset states:
It remains amazing that contemporary Luther biographies are still being written without ever mentioning the name and significance of Bernard of Clairvaux, or with just a hint at him in passing as if it were a chose negligeable. From now on, Luther biographies, if they want to present the authentic, historical Luther, can be convincing only if the Bernard factor is properly figured in, because the center piece of the historical Luther's doctrine of justification is identical with the one of Saint Bernard. (p. 127).
Posset also notes Melanchthon included a conflated Bernard quote in his Apology of the Augsburg Confession:
Therefore let pious consciences know that God commands them to believe that they are forgiven freely on account of Christ and not on account of our works. Let them sustain themselves with this command of God against despair and against the terrors of sin and death. Let them know that this position has been extant among the saints in the church since the beginning of the world. For Peter clearly cites the consensus of the prophets, and the writings of the apostles bear witness that they hold the same position. Nor do we lack testimonies from the Fathers. For Bernard says the same thing in words that are not at all obscure: "For it is necessary first to believe that you cannot have forgiveness of sins except through God's indulgence; second, that you cannot claim any good work whatever unless He himself grants it to you; finally, that you cannot merit eternal life by any works of yours, unless it too is given to you gratis [Ann 1.1]; but add to this that you also believe that through him [Christ] your sins are forgiven you [tibi]. This is the testimony that the Holy Spirit gives in your heart [Rom 8:16], saying, `your sins are forgiven you' [Mt 9:5]. For in this way the Apostle concludes that `a person is gratis justified through faith' [Rom 3:24, 281" [Ann 1.3]. Bernard's words wonderfully shed light on our position, because he not only requires that we believe in a general way that sins are remitted through mercy, but he urges that we add personal faith by which we believe that our own sins have been forgiven. Moreover, he teaches us how we may be certain about the forgiveness of sins, namely, when by faith our hearts are uplifted and find rest through the Holy Spirit. What more do our opponents need? Do they still dare to deny that we receive the forgiveness of sins by faith or that faith is a part of repentance? (p. 136-137)
Posset includes a relevant section of Ann 1 in his book, Two-Fold Knowledge: Readings on the Knowledge of Self and the Knowledge of God-Selected & Translated From The Works Of Saint Bernard Of Clairvaux (pp. 118-120):