In the Second Temple period.
Reprinted with permission from Jerusalem: Portrait of the City in the Second Temple Period (Jewish Publication Society).
A notable development in Hasmonean times was the emergence of identifiable religious sects. The term "sect" requires some clarification, as it usually is used in regard to Christian groups that periodically broke way from the Church for social and ideological reasons. In this period, only the Essenes of Qumran come close to fitting that definition.
Other groups, such as the Pharisees, Sadducees, Hasidians of Maccabean days, Sicarii, and early Christians, all operated in Jerusalem and wider Judaean society and were not a priori opposed to the religious establishment. The term "sect" is thus not the most appropriate for our historical context. Nevertheless, we have retained it out of convenience, since it is universally used with reference to these groups.
When Did Second Temple Sects Begin?
All evidence points to the beginning of the Hasmonean era [2nd century BCE] as the time when the number of Second Temple sects crystallized. While some scholars have posited the existence of these sects, or perhaps more accurately proto‑sects, as early as the fourth and third centuries, such theories are entirely speculative as there are no data to substantiate them.
Josephus [historian, soldier and political figure (1st century CE)], however, first mentions the existence of sects in the middle of the second century, and the development of the communal, sectarian center in Qumran can be dated to its latter half [… ] [Qumran, a settlement on the shores of the Dead Sea, served as headquarters for the Essenes in Second Temple times and is currently a major archeological site.]
Why Did Sects Form?
Religious sectarianism was indeed an unusual occurrence within ancient Judaism. Neither before the second century B.C.E. nor after 70 C.E. did the same range of organized sects exist among Jews, and thus the situation that first crystallized under the Hasmoneans was indeed sui generis. The historical circumstances of the middle second century would seem to have been most conducive for spawning such groups. This was indeed a time of transition and upheaval. Jewish society had been coping for decades with the attractiveness, threat, or both of Hellenism, a process […] that culminated in the transformation of Jerusalem into a polis in 175.