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 Yet for all this we do not find the title ἐκκλησία, wholly withdrawn from the Jewish congregation; that too was “the Church in the wilderness” (Acts vii. 38); for Christian and Jewish differed only in degree, and not in kind.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).