The concept of the "Church" has been long debated. Good people differ on exactly what it means and how it is composed, what are its essential characteristics and who are its congregates. I certainly don't believe anything I say could end the arguments. I can only conclude with a summary of where I stand, of what I believe constitutes the True "Church."
First, "The Church" is the universal, invisible, spiritual Body of Christ. It is organic, born of the Holy Spirit of God, not of men.
Second, "The Church" as scripturally understood is to be differentiated from "the local church," a denomination, a sect, or some other man-made creation or invention. These entities are what I refer to as made with hands churches; they do not necessarily reflect the spiritual principles of God's Kingdom and true Body.
Third, Since the definition of ekklesia is "a called out assembly," any group of individuals meeting in Christ for worship, prayer, and study of Scripture in accord with the Spirit and in keeping with core Christian Doctrines, may be Biblically referred to as "The Church."
Fourth, made with hands entities called "churches," denominations, and other such institutiuons and organizations within "Christendom" were the creation of humans. In many cases, they were founded with great and good intentions, but they have become (or are becoming) useful tools to control and exert power, rather than to further God's kingdom. As such, these made with hands churches may only be regarded as "The Church" in proportion to how obedient they are to the Scriptures and the guidance of the Holy Spirit. Many of these entities, unfortunately, must be discarded as apostate, or as a hindrance to the True Church and the True Gospel. This includes not merely liberal and modernist churches and institutions, but also evangelical and fundamental churches and organizations.
Fifth, the fundamental reason for the corruption of the institutional (made with hands) churches is the depravity of man.
NOTE: Sorry I've been absent for a while -- I've been "under the weather."