Initially Christians were a sect within Judaism. Christian was a 'nick-name' given to them because they believed that Jesus was the Messiah (Christos in Greek).
In 89 C.E., after the Jewish authorities had re-grouped following the destruction of the Temple by the Romans in 70 C.E., Christians were proscribed (outlawed) as a sect, and became a separate religious group.
Unlike their fellow Jews, Christians, especially St. Paul, encouraged non-Jews to join them as followers of Jesus. This led to them dropping such Jewish requirements as keeping kosher and circumcision, and they became a distinct religion.