Church of England consultant, Ven. Bob Jackson, proposes an answer to his own question in a recent paper.
“Once upon a time we thought we knew what church was and how to measure it. Church happened when we gathered in a consecrated building for a public act of worship with a priest on a Sunday. So we measured the size of the church by the number of people who attended the public act of worship. Until the year 2000 we counted ‘Usual Sunday Attendance’, and since then we’ve also used ‘Average weekly attendance in October’, including weekdays.
But attendance & electoral roll measures have never done full justice to what we think church really is. So today I want to pose the deeper questions: ‘What is church?’ & ‘How do we measure it?’”