Connelly, a historian of Central and East-Central Europe and professor of history at the University of California at Berkeley, traces the complicated progression from the church’s long anti-Semitism and persecution of Jewish people to its recognition of Jews as brothers and sisters created in God’s image.

Investigating the progression of political, social and religious events, scientific, scriptural and theological research, and individual relationships across countries and traditions of faith, Connelly presents a dense story, noting in his introduction: “Rather than force linear elegance upon a crooked historical path, the narrative that follows occasionally pauses to wonder about ideas that led nowhere or roads that were not taken.”

The first chapters provide the background for the hatred and suspicion in pre- and interwar Europe. Understandings of race, identity and the natural order were given different emphases in different places in these years, according to Connelly. “Catholicism can vary significantly across boundaries, and some national variants proved more open to racism than others.”

And while he notes that church leaders proceeded with caution when the science of the times upheld “race” as a biological reality, the German cultural climate and its mixing of science with theology was not without its influence on the church. One avowedly anti-Nazi Catholic preacher could still declare that “baptism was powerless to cure Jews of moral defects that they carried in their genes.”

The book’s middle chapters introduce the politically active Catholics outside of the Vatican, and living outside of Germany. A thread throughout the narrative is the role played by a small group of Catholic converts, most of whom were born Jewish. Connelly provides the history showing that “virtually every figure of note in the Catholic battle against anti-Semitism was a convert.”

Their study of the Christian tradition and of modern science provided the materials that refocused intellectual and theological debates across international and ecclesial circles. They fought to uphold a new vision of understanding and reconciliation between Catholics and Jews, and of cooperation with Protestants and Jews. They practiced “de facto ecumenism, an extraordinary phenomenon on the European continent of that time. … But because these Catholics were converts it was difficult to tell them to shun contacts with the outside. The outside, after all, was their homeland.”

These converts, “border-crossers” as Connelly calls them, were still considered alien in the Catholic Church since they were considered racially Jewish. These encounters with racist Christianity in Germany prompted their work to “convert their new Christian world to a true lost faith; to move discussions about race and anti-Semitism beyond stale, self-contradictory patterns; to return to original texts and bring the fold back to original understandings.”

How the church is brought back to “original understandings” is the material of the book’s last chapters, as the groundwork for Vatican II is laid, the church’s relation to the Jews is reframed, and the painstaking crafting of “Nostra Aetate” is accomplished. “By answering the question ‘Who are the Jews?’ the Catholic Church had found its way across previously insurmountable boundaries to tolerance, to recognizing that God extends grace to all humans.”

“From Enemy to Brother” is an exceptional resource, deeply thoughtful, carefully written and extensively documented. Making one’s way through the 300 pages of text, with another 60 pages of endnotes, is a worthy project.

This nuanced and extraordinary new entry into Catholicism’s past might serve to strengthen the orientation of its future, and bolster an unwavering commitment to this statement from “Nostra Aetate”: “Therefore, the church reproves, as foreign to the mind of Christ, any discrimination against people or any harassment of them on the basis of their race, color, condition in life or religion.”

Kantor, who lives in Boston, is a writer, lecturer and adjunct faculty member at area colleges and holds a doctorate in religion and society from Harvard Divinity School.