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.  

Many in the professional clergy understand their role as a "priestly" one in which they are to be intermediaries between the Lord and His flock, being thus trusted through the Ordination process with a certain degree of authority.

Institutions tend toward viewing its members as an "audience" and the worship experience as a show. It is better to view God as the audience and all the people equally accountable for the "performance" of worshipping in Spirit and in Truth.

Three reasons for house meetings

*  All gatherings in the young church era were small assemblies that met in homes.  

*  For non-denominational Christians an invitation offered to a work-place acquaintance to a home is much less threatening than one to a church.

*  House gatherings reject any human authority other than the very real and present rule of Christ, who was inaugurated the king at the first Pentecost (Acts 2). The house gathering assembles are supposed to know the will of its king through the Holy Spirit and to be obedient to that will. As long as they  are focusing on the evangelical message of equal treatment, freedom from authority and helpfulness, they are bound.

These arguments have been taken from House Church Central