When was Christianity born? When was it that Christianity, born as a particular current within Judaism, constituted itself as a religion different and separate from the Jewish religion? The question has been asked, and the problem has therefore been considered, since the historical-critical investigation of Christian origins began. However the problem has become acute only in the last few decades, because of the occurrence of a whole series of circumstances and of reflections that have deeply changed the historiographic understanding regarding Judaism in the first century, and thus the origins of Christianity as well. Traditional opinion considered the founders of Christianity to be Jesus of Nazareth and Paul of Tarsus. Recent studies however affirm that a Christian religion as distinct from the Jewish religion can be spoken of only much later, and that for the entire first century, and for at least a part of the second century, Christianity was nothing more than a sect within Judaism. Dealing with the problem from an historical point of view, and thus considering not only Christianity of Jewish origin but also that of gentile origin, Giorgio Jossa demonstrates that the birth of a Christian identity as distinct from Jewish identity must actually be dated back to the first period of life of the community of Jesus.