What church?

What defines a “Christian” church?  There are foundational doctrines and secondary ones.  I think all would agree that the secondary ones are not part of this consideration.  First, who decides what is foundational?  There are multiple thousands of “Christian” denominations in the US right now, many of them solo with no oversight other than the single person that created them.  The common mentality seems to be that anyone can take the label “Christian” upon themselves which in turn gives them the right to interpret the Bible for themselves.
When we give a definition of “church”, from where are we sourcing that definition?  In Protestant and Evangelical circles, the claim is that the definition comes from the Bible. Yet there are multiple definitions, all with the same claim of origin.  But only one can be right, or they are all wrong. This circles back around to the practice of personal interpretation.

This is one strength of the Orthodox church.  We can look back and say, "this is what the church has always believed and always practiced. This is how the church has always worshiped."  In short, we can say, the church is what the body of Christ has always believed the church to be.