Mt. 18:20 is the source of the house meetings doctrine.
is "Where two or three are gathered in my name, there am I in the midst of them."
The "glue" that is necessary to unite believers cannot be achieved as a meeting grows beyond a limit. Some experts point out that even an assembly larger than a mere dozen people creates an environment in which some of the people often back away from full participation.
This argument has been taken from Home Church Central
HCC is a non-denominational protestant guide for worshipping in small groups