Josephus reports four main sects or schools of Judaism: Pharisees, Sadducees, Essenes, and Zealots. The earliest followers of Jesus were known as Nazarenes, and perhaps later, Ebionites, and form an important part of the picture of Palestinian Jewish groups in late 2nd Temple times.
The Ebionite/Nazarene movement was made up of mostly Jewish/Israelite followers of John the Baptizer and later Jesus, who were concentrated in Palestine and surrounding regions and led by “James the Just” (the oldest brother of Jesus), and flourished between the years 30-80 C.E. They were zealous for the Torah and continued to walk in all the mitzvot (commandments) as enlightened by their Rabbi and Teacher, but accepted non-Jews into their fellowship on the basis of some version of the Noachide Laws (Acts 15 and 21). The term Ebionite (from Hebrew ‘Evyonim) means “Poor Ones” and was taken from the teachings of Jesus: “Blessed are you Poor Ones, for yours is the Kingdom of God” based on Isaiah 66:2 and other related texts that address a remnant group of faithful ones. Nazarene comes from the Hebrew word Netzer (drawn from Isaiah 11:1) and means “a Branch”—so the Nazarenes were the “Branchites” or followers of the one they believed to be the Branch. The term Nazarene was likely the one first used for these followers of Jesus, as evidenced by Acts 24:5 where Paul is called “the ringleader of the sect of the Nazarenes.” Here we see the word used in a similar way to that of Josephus in writing of the four sects/schools of Judaism: Pharisees; Sadducess; Essenes; and Zealots. So the term Nazarene is probably the best and broadest term for the movement, while Ebionite (Poor Ones) was used as well, along with a whole list of other terms: Saints, Children of Light, the Way, New Covenanters, et al. We also know from the book of Acts that the group itself preferred the designation “The Way” (see Acts 24:14, 22, etc.). The term “Christian,” first used in Greek speaking areas for the movement, actually is an attempt to translate the term Nazarene and basically means a “Messianist.”
The Essenes (possibly from ‘Ossim, meaning “Doers of Torah”), who wrote or collected the Dead Sea Scrolls, pioneered certain aspects of this “Way” over 150 years before the birth of Jesus. They were a wilderness (out in the Arava, near the Dead Sea–based on Isaiah 40:3), baptizing (mikveh of repentance as entrance requirement into their fellowship), new covenant, messianic/apocalyptic group (they were expecting three redemptive Figures—the Prophet like Moses and his two Messiahs), that saw themselves as the remnant core of God’s faithful people—preparing the Way for the return of YHVH’s Glory (Kavod) as set forth in Isaiah 40-66. They too referred to themselves as the Way, the Poor, the Saints, the New Covenanters, Children of Light, and so forth. Perhaps their most common designation was the Yachad–the brotherhood or community, and they referred to themselves as brother and sister. They were bitterly opposed to the corrupt Priests in Jerusalem, to the Herods, and even to the Pharisees whom they saw as compromising with that establishment to get power and influence from the Hellenistic/Roman powers. They had their own developed Halacha (interpretation of Torah), some aspects of which Jesus picks up (ideal of no divorce, not using oaths, etc.). They followed one they called the True Teacher (Teacher of Righteousness) whom most scholars believe lived in the 1st century B.C.E. and was opposed and possibly killed by the Hasmonean King/Priests at the instigation of the Pharisees. John the Baptizer seems to arise out of this context and rekindle the apocalyptic fervor of the movement in the early decades of the first century C.E.
So, the terminology is flexible; there are a variety of self-designations used by the Jesus movement, most of which had previously been used by the Essenes. In that sense you might call the Jesus movement a further developed messianic “Essenism,” modified through the powerful, prophetic influence of Jesus as Teacher.
Later, when Christianity developed in the 3rd and 4th centuries and gradually lost its Jewish roots and heritage, largely severing its Palestinian connections, the Gentile, Roman Catholic Church historians began to refer to Ebionites and Nazarenes as two separate groups—and indeed, by the late 2nd century there might have been a split between these mostly Jewish followers of Jesus. The distinction these writers make (and remember, they universally despise these people and call them “Judaizers”), is that the Ebionites reject Paul and the doctrine of the Virgin Birth or “divinity” of Jesus, use only the Hebrew Gospel of Matthew, and are thus more extreme in their Judaism. They describe the Nazarenes more positively as those who accept Paul (with caution) and believe in some aspect of the divinity of Jesus (virgin born, etc.). What we have to keep in mind in reading these accounts from the Church fathers is that they are strongly prejudiced against this group(s) and claim to have replaced Judaism entirely with the new religion of Christianity, overthrowing the Torah for both Gentile and Jew.
I think it best today to use the collective term Ebionite/Nazarene in an attempt to capture the whole of this earliest movement, and it would be useful to revive the term Yachad as a collective designation for the community of the Hasidim/Saints. I use Ebionite/Nazarene as an historical designation to refer to those original, 1st century, largely Palestinian followers of Jesus, gathered around Yaaqov (James) in Jerusalem, who were zealous for the Torah, but saw themselves as part of the New Covenant Way inaugurated by their “True Teacher” Jesus. James is a key and neglected figure in this whole picture. As the blood brother of Jesus, authority and rights of guidance were passed on to him. When he was brutally murdered in 62 C.E. by the High Priest Ananus (see Josephus, Antiquities 20.197ff), Simeon, a second brother [sic “cousin” according to Hegesippus] of Jesus took over the leadership of the Jerusalem based movement. Clearly we have the idea here of a blood-line dynasty, and according to the Gospel of Thomas, discovered in 1946 in upper Egypt, this dynastic succession was ordained by Jesus himself who tells his followers who ask him who will lead them when he leaves: “No matter where you are, you are to go to James the Just, for whose sake heaven and earth came into being” (GT 12). Indeed, when Simeon was crucified by the Emperor Trajan around 106 C.E., a third brother of Jesus, Judas, took over the leadership of the community.
As far as “beliefs” of the Ebionites, the documents of the New Testament, critically evaluated, are among our best sources. There are fragments and quotations surviving from their Hebrew Gospel tradition (see see A. F. J. Klijn, Jewish-Christian Gospel Tradition, E. J. Brill, 1992), as well as the text of “Hebrew Matthew” preserved by Ibn Shaprut, and now published in a critical edition by George Howard (The Hebrew Gospel of Matthew, Mercer University Press, 1995). Based on what we can reliably put together from other sources we can say the Ebionite/Nazarene movement could be distinguished by the following views:
1) Jesus as the Prophet like Moses, or True Teacher (but not to be confused with YHVH God of Israel), who will anoint his Messiahs on his right and left hand when he is revealed in power following his rejection and death. These two figures, the Davidic Nasi (Prince of the Yachad) and Priest, will rule with him in the Kingdom of God.
2) Disdain for eating meat and even the Temple slaughter of animals, preferring the ideals of the pre-Flood diet and what they took to be the original ideal of worship (see Gen 9:1-5; Jer 7:21-22; Isa 11:9; 66:1-4). A general interest in seeking the Path reflected in the pre-Sinai revelation, especially the time from Enoch to Noah. For example, divorce was shunned, even though technically it was later allowed by Moses.
3) Dedication to following the whole Torah, as applicable to Israel and to Gentiles, but through the “easy yoke” halacha of their Teacher Jesus, which emphasized the Spirit of the Biblical Prophets in a restoration of the “True Faith,” the Ancient Paths (Jeremiah 6:16), from which, by and large, they believed the establishment Jewish groups of 2nd Temple times had lost.
4) Rejection of the “doctrines and traditions” of men, which they believed had been added to the pure Torah of Moses, including scribal alterations of the texts of Scripture (Jeremiah 8:8).
How the earliest group(s) viewed Paul is unclear. By some reports he was tolerated or accepted as one who could go to the Gentiles with a version of the Nazarene message (Acts 15, 21). Others apparently believed he was an apostate from the Torah and founder of a new religion—Christianity.
For further reading, see H-J Schoeps, Jewish Christianity (Philadelphia: Fortress Press, 1969), still useful and quite readable for students.