Were gathered together (συνηχθησαν). First aorist (effective) passive indicative. The church is not named here as in verse Acts 15:4, but we know from verses Acts 15:12-22 that the whole church came together this time also along with the apostles and elders.

Of this matter (περ του λογου τουτου). Same idiom in Acts 8:21; Acts 19:38. They realized the importance of the issue.