There is an interesting discussion by Vitringa (De Synag. Vet. pp. 77-89) on the distinction between these two Hebrew synonyms; the result of which is summed up in the following statements: ‘Notat proprie lhq universam alicujus populi multitudinem, vinculis societatis unitam et rempublicam sive civitatem quondam Constituentem, cum vocabulum hdf ex indole et vi significationis sage tantum dicat quemcunque hominum coetum et conventum, sive minorem sive majorem’ (p. 80). And again: ‘Συναγωγή, ut et hdf, semper significat coetum conjunctum et congregatum, etiamsi nullo forte vinculo ligatum, sed ἡ ἐκκλησία [=lhq] designat multitudinem aliquam; (quae populum constituit, per leges et vincula inter se junctam, etsi saepe fiat non sit coacta vel cogi possit’ (p. 88).
Accepting this as a true distinction, we shall see that it was not without due reason that our Lord (Matt. xvi. 18; xviii. 17) and his Apostles claimed this, as the nobler word, to designate the new society of which He was the Founder, being as it was a society knit together by the closest spiritual bonds, and altogether independent of space. Yet for all this we do not find the title ἐκκλησία, wholly withdrawn from the Jewish congregation; that too was “the Church in the wilderness” (Acts vii. 38); for Christian and Jewish differed only in degree, and not in kind. Nor yet do we find συναγωγὴ wholly renounced by the Church; the latest honorable use of it in the New Testament, indeed the only Christian use of it there, is by that Apostle to whom it was especially given to maintain unbroken to the latest possible moment the outward bonds connecting the Synagogue and the Church, namely, by St. James (ii. 2); ἐπισυναγωγή, I may add, on two occasions is honorably used, but in a more general sense (2 Thess. ii.1; Heb. x. 25).
Occasionally also in the early Fathers, in Ignatius for instance (Ep. ad Polyc. 4; for other examples see Suicer, s. v.), we find συναγωγὴ still employed as an honorable designation of the Church, or of her places of assembly. Still there were causes at work, which led the faithful to have less and less pleasure in the appropriation of this name to themselves; and in the end to leave it altogether to those, whom in the latest book of the canon the Lord had characterized for their fierce opposition to the truth even as “the synagogue of Satan” (Rev. iii. 9; cf. John viii. 4).
Thus the greater fitness and dignity of the title ἐκκλησία has been already noted. Add to this that the Church was ever rooting itself more predominantly in the soil of the heathen world, breaking off more entirely from its Jewish stock and stem. This of itself would have led the faithful to the letting fall of συναγωγή, a word with no such honorable history to look back on, and permanently associated with Jewish worship, and to the ever more exclusive appropriation to themselves of ἐκκλησία, so familiar already, and of so honorable a significance, in Greek ears.
It is worthy of note that the Ebionites, in reality a Jewish sect, though they had found their way for a while into the Christian Church, should have acknowledged the rightfulness of this distribution of terms. Epiphanius (Haeres. xxx. 18) reports of these, συναγωγὴν δὲ οὗτοι καλοῦσιν τὴν ἑαυτῶν ἐκκλησίαν, καὶ οὐχὶ ἐκκλησίαν.
It will be perceived from what has been said, that Augustine, by a piece of good fortune which he had no right to expect, was only half in the wrong, when transferring his Latin etymologies to the Greek and Hebrew, and not pausing to enquire whether they would hold good there, as was improbable enough, he finds the reason for attributing συναγωγὴ to the Jewish, and ἐκκλησία to the Christian Church, in the fact that ‘convocatio’ (=ἐκκλησία) is a nobler term than ‘congregatio’ (=συναγωγή), the first being properly the calling together of men, the second the gathering together (‘congregatio,’ from ‘congrego,’ and that from ‘grex’) of cattle. See Field, On the Church, i. 5.
