16 Better to pray together than separately.
But here again I would ask my accuser, where was it right that the people should pray? In the deserts, or in a place which was in course of building for the purpose of prayer? Where was it becoming and pious that the people should answer, Amen? In the deserts, or in what was already called the Lord's house? Where would you, most religious Prince, have wished your people to stretch forth their hands, and to pray for you? Where Greeks, as they passed by, might stop and listen, or in a place named after yourself, which all men have long called the Lord's house, even since the foundations of it were laid? I am sure that you prefer your own place; for you smile, and that tells me so. 'But,' says the accuser, 'it ought to have been in the Churches. They were all, as I said before, too small and confined to admit the multitude. Then again, in which way was it most becoming that their prayers should be made? Should they meet together in parts and separate companies, with danger from the crowded state of the congregation? Or, when there was now a place that would contain them all, should they assemble in it, and speak as with one and the same voice in perfect harmony? This was the better course, for this showed the unanimity of the multitude: in this way God will readily hear prayer. For if, according to the promise of our Saviour Himself, where two shall agree together as touching anything that they shall ask, it shall be done for them, how shall it be when so great an assembly of people with one voice utter their Amen to God? Who indeed was there that did not marvel at the sight? Who but pronounced you happy when they saw so great a multitude met together in one place? How did the people themselves rejoice to see each other, having been accustomed heretofore to assemble in separate places! The circumstance was a source of pleasure to all; of vexation to the calumniator alone.
Source: Apologia ad Constantium (New Advent)