[1] Internationally, Isaac was most well known for his tireless work after the Second World War in the field of Jewish-Christian relations, starting with his book Jésus et Israël, which was written during the war and made him a protagonist in the Seelisberg Conference of 1947, culminating in his decisive key role in the origin of the groundbreaking declaration Nostra Aetate during the Second Vatican Council.

"[12] As a writer of history texts, known as Malet-Isaac, Isaac was closely associated with the Hachette Book Group.

The publisher feared that Isaac's Jewish name would hinder sales of the texts to Catholic schools.

"[25] Isaac's textbooks clarified historical events and "brought students into contact with the original source materials.

"[26] When World War II broke out with the German invasion of Poland beginning on September 1, 1939, Isaac was a "highly respected historian in France.

[28] Within four months, the Act of October 3, 1940, of the German-controlled Vichy government began "a series of discriminatory laws against Jews.

[34] When the Free Zone was occupied, Isaac moved first to Le Chambon-sur-Lignon, then to Riom, near his daughter and his son-in-law.

[35] On October 7, 1943, Isaac's wife, daughter, son-in-law, and one of his sons were arrested in Riom by the German Gestapo.

[38] The fact of the anti-Jewish measures of the Nazis coupled with the lack of condemnation by Christians made Isaac wonder what were the roots of anti-Semitism.

At the end of 1942, using books provided by clergy who were his friends, Isaac began researching the Gospels and "the classic theological Christian tradition on the Jews."

[46] Isaac was instrumental in the 1950 founding of the Amicizia Ebraico-cristiana di Firenze (Jewish Christian Friendship Association).

[47] Isaac's first book based on his research regarding the Christian roots of anti-Semitism was Jésus et Israël (published in 1948, and translated into English in 1971 as Jesus and Israel).

The book is "a 600 page analysis of anti-Semitism and Christianity which compared the texts of the Gospels with Catholic and Protestant scriptural commentaries conveying a distorted picture of Jesus' attitude toward Israel and Israel's attitude toward Jesus, and which he believed were largely responsible for the anti-semitic conditioning of European Christians.

[52] Near the end of the book, Isaac urged Christians to "recognize their initial responsibility" for anti-Judaism and to expunge such beliefs from their hearts and minds and engage in a "strenuous examination of conscience."

He added that for him such action by Christian was urgent because "the glow of the Auschwitz crematorium is the beacon that lights, that guides all my thoughts.

[54] In an Appendix to Jesus and Israel, Isaac included his Eighteen Points needed to rectify Christian doctrine.

[58] In reply to such critics, in 1956 Isaac wrote Génése de l’Anti-Sémitisme (Translated into English as Has Anti-Semitism Roots in Christianity?

Christianity "added theology to historical xenophobia" and condemned Jews "as a people of deicides to be cursed, punished, driven into exile."

Isaac expanded this argument in his final book published a year before his death, L'Enseignement de Mépris (1962).

"[61] In the English translation, Isaac said, "we are all familiar with the words of Jesus from the Fourth Gospel, ‘In my Father's house are many mansions’ (John 14:2).

[65]This section is about three critical meetings in which Isaac (sometimes and other Jewish leaders) had with Roman Catholic officials that effected changes in the Church's attitude toward Jews.

During his audience, Isaac "summarized in a portfolio his research into the history of the Christian ‘teaching of contempt’ for Jews and Judaism.

"[70] Isaac's diary entries regarding his preparation and the audience with Pope John XXIII can be read at “Isaac’s Audience with Pope John XXIII.” The audience was a major impetus for Pope John's directing Cardinal Augustin Bea, the president of the Pontifical Council for Promoting Christian Unity, to draft "a declaration on the Catholic Church's relationship to the Jewish people for the upcoming Second Vatican Council."

[71] "With Nostra Aetate the Catholic Church took leave from an almost 2,000 year old anti-Jewish tradition, a profound break with the age-old teaching of contempt.

"[72] On January 6, 1956, at the Hôtel Lutetia in Paris (where survivors of the extermination camps had come after their release to be met by their families), the MRAP (NGO) was awarded to Isaac.

The award was in recognition of the "great impact" of Isaac's books Jésus et Israël (Jesus and Israel) and Genèse de l'antisémitisme (Genesis of Antisemitism).

Although he, like Pope John XXIII, did not live to see the results of the Vatican II Council in Nostra Aetate, published in 1965, this document was the culmination of Isaac’s work in Jewish-Christian relations and his voice "could be heard echoing" in it.

[75] An International Conference to commemorate the fiftieth anniversary of Isaac's death was held June 30–July 3, 2013, in Aix-en-Provence.

The opening address “The Legacy of Jules Isaac” Archived 2016-10-07 at the Wayback Machine was given by Edouard Robberechts who is a lecturer in Jewish philosophy at the Aix-Marseille University.

[79] The Foundation's goals include furthering knowledge about the life and works of Isaac, in particular with regard to his role as a pioneer in the development of a Christian theology without anti-Semitism.