Matthew 18:20

δύο ἢ τρεῖς. In the smallest gathering of His followers Christ will be present.

συνηγμένοι. συνάγειν is used specially of the ‘gathering’ of the Church, as Acts 11:26, συναχθῆναι ἐν τῇ ἐκκλησίᾳ. Matthew 20:8, ἐν τῷ ὑπερῴῳ οὗ ἦμεν συνηγμένοι. Hence in later Ecclesiastical Greek σύναξις is ‘a religious service,’ συναξάριον a ‘service book,’ συνάξιμος ἡμέρα ‘a day on which services are held.’