Unity of the brethren, as expressed by Jesus Christ (Jhn 10:16, Jhn 17:11, Jhn 17:21-22), 
is not to be confused with a general lumping together of any group that calls itself "Christian" or a Religion.

There is but one body (1Cr 6:16) of Christ, not many.  There are many Christian Churches, but true Christians are all unified as one body, regardless of their location and differences.

So what is the definition of a Christian.  Just what defines true Christian unity?

What about those "Christians" who have different doctrines (teachings) from "us"?     Aren't they "Christians"?

What does the scripture say?   Act 2:42 And they continued steadfastly in the apostles' doctrine and fellowship, and in breaking of bread, and in prayers.

OK, but what teachings are "the apostle's doctrine"?   Just how far away from my beliefs can someone get and still be called a Christian?     

Well, let's see what is actual "doctrine".