In The Church History of Eusebius, NPNF2-01 Bk. III, Chapter XXVII: The heresy of the Ebionites (available online at http://www.ccel.org/ccel/schaff/npnf201.iii.viii.xxvii.html). Footnotes 824 states:
The Ebionites were not originally heretics. Their characteristic was the more or less strict insistence upon the observance of the Jewish law; a matter of cultus, therefore, not of theology, separated them from Gentile Christians. Among the early Jewish Christians existed all shades of opinion, in regard to the relation of the law and the Gospel, from the freest recognition of the uncircumcised Gentile Christian to the bitterest insistence upon the necessity for salvation of full observance of the Jewish law by Gentile as well as by Jewish Christians. With the latter Paul himself had to contend, and as time went on, and Christianity spread more and more among the Gentiles, the breach only became wider. In the time of Justin there were two opposite tendencies among such Christians as still observed the Jewish law: some wished to impose it upon all Christians; others confined it to themselves. Upon the latter Justin looks with charity; but the former he condemns as schismatics (see Dial. c. Trypho. 47). For Justin the distinguishing mark of such schismatics is not a doctrinal heresy, but an anti-Christian principle of life. But the natural result of these Judaizing tendencies and of the involved hostility to the apostle of the Gentiles was the ever more tenacious clinging to the Jewish idea of the Messiah; and as the Church, in its strife with Gnosticism, laid an ever-increasing stress upon Christology, the difference in this respect between itself and these Jewish Christians became ever more apparent until finally left far behind by the Church in its rapid development, they were looked upon as heretics. And so in Irenæus (I. 26. 2) we find a definite heretical sect called Ebionites, whose Christology is like that of Cerinthus and Carpocrates, who reject the apostle Paul, use the Gospel of Matthew only, and still cling to the observance of the Jewish law; but the distinction which Justin draws between the milder and stricter class is no longer drawn: all are classed together in the ranks of heretics, because of their heretical Christology (cf. ibid. III. 21. 1; IV. 33. 4; V. 1. 3). In Tertullian and Hippolytus their deviation from the orthodox Christology is still more clearly emphasized, and their relation to the Jewish law drops still further into the background (cf. Hippolytus, Phil. VII. 22; X. 18; and Tertullian, De Carne Christi, 14, 18, &c.). So Origen is acquainted with the Ebionites as an heretical sect, but, with a more exact knowledge of them than was possessed by Irenæus who lived far away from their chief centre, he distinguishes two classes; but the distinction is made upon Christological lines, and is very different from that drawn by Justin. This distinction of Origen’s between those Ebionites who accepted and those who denied the supernatural birth of Christ is drawn also by Eusebius (see below, §3). Epiphanius (Hær. XXIX. sqq.) is the first to make two distinct heretical sects—the Ebionites and the Nazarenes. It has been the custom of historians to carry this distinction back into apostolic times, and to trace down to the time of Epiphanius the continuous existence of a milder party—the Nazarenes—and of a stricter party—the Ebionites; but this distinction Nitzsch (Dogmengesch. p. 37 sqq.) has shown to be entirely groundless. The division which Epiphanius makes is different from that of Justin, as well as from that of Origen and Eusebius; in fact, it is doubtful if he himself had any clear knowledge of a distinction, his reports are so contradictory. The Ebionites known to him were most pronounced heretics; but he had heard of others who were said to be less heretical, and the conclusion that they formed another sect was most natural. Jerome’s use of the two words is fluctuating; but it is clear enough that they were not looked upon by him as two distinct sects. The word “Nazarenes” was, in fact, in the beginning a general name given to the Christians of Palestine by the Jews (cf. Acts xxiv. 5), and as such synonymous with “Ebionites.” Upon the later syncretistic Ebionism, see Bk. VI. chap. 38, note 1. Upon the general subject of Ebionism, see especially Nitzsch, ibid., and Harnack, Dogmengeschichte, I. p. 226 sqq.
Thus, it can be seen that the Ebionites came from the Jerusalem Church which had apostatized as they cave in under Jewish pressure, plus the fact that the Church there refuses to mature into the fullness of the Christian faith (especially with regards to Christology it seems) but continue to identify Christianity as a Jewish faith, failing to differentiate between the true Abrahamic and Mosaic Judaism as compared to the distorted rabbinic Judaism of their time.