The Community of those who follow Jesus
The word "church" (Greek "ekklesia") is a much broader word than it has come to be in contemporary usage, particularly since the word is now used also to mean a specialized "building" - something not conceived of in the New Testament.
In the New Testament "ekklesia" is used to speak of four kinds of groups:
the nation of Israel as a whole
a group of people gathered for a town meeting (e.g. Acts 19: 32, 39, 41)
all believers who are in Christ, sometimes called "the church universal" (e.g. Col. 1:18)
a local group of professing Christians, e.g. the church at Antioch (Acts 13:1)
The common element linking each of these expressions is that of being "called out" to participate in a specific purpose.
To follow Christ is to be "called out" of the world to participate in the mission of God in the world. This is done in community and the New Testament views every believer as normally an active part of it.