Promissio is a methodologically rigorous study of all the pertinent Luther texts prior to 1520, thus enabling Bayer to trace Luther's reformational turn, culminating in his clear understanding of the gospel promise as God's gift, crystallized in the forgiveness of sins and enacted by the word of absolution. The author emphasizes that at the heart of Luther's understanding of the gospel is the insight that God justifies us freely through faith in the word of promise (promissio) and that this promise is not oriented primarily to the future but is a present gift to be received in faith. However, he shows that it is not the understanding of justification by faith that is at the center of Luther's breakthrough, but the insight that God's promise is an efficacious word that accomplishes what it says, and that justification can only be properly understood in the light of the promise.

Bayer shows how Luther comes to see that the promise requires a reshaping of the sacrament of penance (confession and absolution), as well as Baptism and the Lord's Supper. Luther's new understanding of promise also leads him to radically rethink his understanding of Christology, the sermon, meditation, and prayer.

When this book was first published in 1971 it was hailed by the Roman Catholic scholar Otto Hermann Pesch as a "classic of Luther research," and that claim still holds good today. No serious Luther scholar can afford to ignore it. It has also remained the foundation of all Oswald Bayer's subsequent work. The Lutheran Quarterly Books edition of Promissio now offers scholars and students this wisdom in English, thanks to Jeffrey Silcock's careful translation.