Zealotry was a movement in first century Judaism, described by Josephus as one of the "four sects" at this time. The term Zealot, in Hebrew kanai (קנאי, frequently used in plural form, קנאים), means one who is zealous on behalf of God. The term derives from Greek ζηλωτής (zelotes), "emulator, zealous admirer or follower. The Zealots were a religious group and were frequently in rebellion.
Josephus's Jewish Antiquities states that there were three main Jewish sects at this time, the Pharisees, the Sadducees, and the Essenes. The Zealots were a "fourth sect", founded by Judas of Galilee (also called Judas of Gamala) and Zadok the Pharisee in the year 6 against Quirinius' tax reform, shortly after the Roman state declared what had most recently been the territory of the tribe of Judah a Roman Province, and that they "agree in all other things with the Pharisaic notions; but they have an inviolable attachment to liberty, and say that God is to be their only Ruler and Lord." (18.1.6)
According to the Jewish Encyclopedia article on Zealots:
Following Josephus ("B. J." ii. 8, § 1; "Ant." xviii. 1, §§ 1, 6), most writers consider that the Zealots were a so-called fourth party founded by Judas the Galilean (see Grätz, "Gesch." iii. 252, 259; Schürer, "Gesch." 1st ed., i. 3, 486). This view is contradicted, however, by the fact that Hezekiah, the father of Judas the Galilean, had an organized band of so-called "robbers" which made war against the Idumean Herod ("B. J." i. 10, § 5; "Ant." xiv. 9, § 2), and also during the reign of Herod, if not long before by the fact that the system of religious and political murders practised by the Zealots was in existence during the reign of Herod, if not long before...
In either case, it has also been argued that the group was not so clearly marked out (before the first war of 66-70/3) as some have thought.
The Crisis under Caligula (37-41) has been proposed as the first open break between Rome and the Jews.
The Zealots had the leading role in the Jewish Revolt of 66. They succeeded in taking over Jerusalem, and held it until 70, when the son of Roman Emperor Vespasian, Titus, retook the city and destroyed Herod's Temple during the destruction of Jerusalem.
The Zealots objected to Roman rule and sought violently to eradicate it; Zealots engaged in violence were called the Sicarii. They raided Jewish habitations and killed Jews they considered collaborators, while also urging Jews to fight Romans and other Jews for the cause. Josephus paints a very bleak picture of their activities as they instituted what he characterized as a murderous "reign of terror" prior to the Jewish Temple's destruction.
According to Josephus, the Zealots followed John of Gischala, who had fought the Romans in Galilee, escaped, came to Jerusalem, and then inspired the locals to a fanatical position that led to the Temple's destruction.
The Zealots advocated violence against the Romans and their Sadducee Jewish collaborators, raiding for provisions and other activities that aided their cause.
After the destruction of Jerusalem and the Second Temple in AD 70, 960 Zealots took refuge by capturing the Roman fortress of Masada and taking no prisoners. Rome sent the Tenth Legion to attempt to retake the stronghold, but for three years they met with no success. It is estimated that they took over 1,000 casualties in the process. The Zealots continued to hold the fortress even after the Romans invented new types of siege engines. Finally, in the third year of the siege, Rome, unable to take the fortress intact, gave up and burned the walls down. When the Romans stormed in to capture the Zealots, they found that the fighters and their families had nearly all committed suicide rather than live a life of slavery.
One of their leaders, Elazar ben Yair escaped to the desert fortress of Masada and fought alongside the Sicarii Zealots until Masada was captured in 73. The Jewish Revolt was suppressed thereafter and the Zealots declined in power and finally faded into history
The main differences between the Sicarii and the Zealots were: (1) the Jerusalem Zealots never attached themselves to one particular family and never proclaimed any of their leaders king; (2) the Sicarii had their original base in Galilee, while the Zealots were concentrated in Jerusalem; and (3) the Galilean Sicarii were fighting for a social revolution, while the Jerusalem Zealots placed less stress on the social aspect.