Christianity began as a movement within Second Temple Judaism, but the two religions gradually diverged over the first few centuries of the Christian era. Today, differences of opinion vary between denominations in both religions, but the most important distinction is Christian acceptance and Jewish non-acceptance of Jesus as the Messiah prophesied in the Hebrew Bible and Jewish tradition. Early Christianity distinguished itself by determining that observance of halakha was not necessary for non-Jewish converts to Christianity. Another major difference is the two religions' conceptions of God. Depending on the denomination followed, the Christian God is either believed to consist of three persons of one essence, with the doctrine of the incarnation of the Son in Jesus being of special importance, or like Judaism, believes in and emphasizes the Oneness of God. Judaism, however, rejects the Christian concept of God in human form. While Christianity recognizes the Hebrew Bible as part of its scriptural canon, Judaism does not recognize the Christian New Testament.

Christianity began as a movement within Second Temple Judaism, but the two religions gradually diverged over the first few centuries of the Christian Era, and the Christian movement perceived itself as distinct from the Jews by the fourth century. Historians continue to debate the dating of Christianity's emergence as a discrete religion apart from Judaism. Philip S. Alexander characterizes the question of when Christianity and Judaism parted company and went their separate ways as "one of those deceptively simple questions which should be approached with great care". According to historian Shaye J. D. Cohen, "the separation of Christianity from Judaism was a process, not an event", in which the church became "more and more gentile, and less and less Jewish". Conversely, various historical events have been proposed as definitive points of separation, including the Council of Jerusalem and the First Council of Nicaea.