Judaism
Christianity was originally a sect of Second Temple Judaism, but the two religions diverged in the first century. The differences between Christianity and Judaism originally centered on whether Jesus was the Jewish Messiah but eventually became irreconcilable. Major differences between the two faiths include the nature of the Messiah, of atonement and sin, the status of God's commandments to Israel, and perhaps most significantly of the nature of God himself. Due to these differences, Judaism traditionally regards Christianity as Shituf or worship of the God of Israel which is not monotheistic. Christianity has traditionally regarded Judaism as obsolete with the invention of Christianity and Jews as a people replaced by the Church, though a Christian belief in dual-covenant theology emerged as a phenomenon following Christian reflection on how their theology influenced the Nazi Holocaust.
Jewish religious movements
The Jewish denominations themselves reject characterization as sects. Sects are traditionally defined as religious subgroups that have broken off from the main body, and this separation usually becomes irreparable over time. Within Judaism, individuals and families often switch affiliation, and individuals are free to marry one another, although the major denominations disagree on who is a Jew. It is not unusual for clergy and Jewish educators trained in one of the liberal denominations to serve in another, and left with no choice, many small Jewish communities combine elements of several movements to achieve a viable level of membership.