It is. Remember, the New Testament was inspired and written with orthodox believers in view, not latter day holders to heresies and novelities. New religions, founded only 400 some years ago, or last week for that matter, and their believers are not in the Church although they might be imperfectly connected with it.
So, going by your statement, who is in this church?
I personally do not belong to a religion, I do whoever belong to the Body Of Christ.