True, and this is why fundamentasl theological unity regarding Christ's teaching is required before we can speak of a single Church. By and large, we have such unity with the Orthodox; we don't with the Protestants.
There is a tendency however for some to exclude all who do not believe exactly the same way as they do, as not being members of the body. But I aver those who feel this way are in error - the members of the body are determined by Christ Himself (John 1,3, 14-17; I Cor 2; Rev 2-3) not by mortal men, doctrines, traditions, etc.