What is the New Testament Definition of a Church

[ZNPaaneah]

What is the church? The church is the composition of the Lord’s many brothers. Individually, we are the Lord’s brothers; corporately, we are the church. The church is the composition of the firstborn Son and the many sons of God. (Witness Lee, The Subjective Truths in the Holy Scriptures, Chapter 3, Section 1)

On another thread this has become a bone of contention.

Matt 18:20 20 for where there are two or three gathered together -- to my name, there am I in the midst of them.'

According to Witness Lee

Suppose that you and others in the city where you live are fed up with Christianity, so you start to meet together separately in the Lord's name. You say, “We give up Christianity; we have had enough of the old system of religion; now we are just meeting by ourselves in the name of the Lord Jesus, assured according to Matthew 18:20 that we have His presence.” We would simply ask you, Is your meeting taking the stand with the true local church in your city? Or is your meeting some isolated thing, something without the church as a standing? If so, your meeting is divisive and not a proper meeting. Do not isolate Matthew 18:20—it must be understood by the context. Read the context, and you will see the right meaning of meeting in the name of the Lord. (How to Meet, Chapter 1, Section 2)

So then the question is this What is the "true local church in your city"?

How do you know what it is?

What does it mean to have a meeting that is "some isolated thing", "something without the church as a standing"?