While I am not exactly sure of who wrote it, I commend to the attention of the about 6 or so of you who read this little corner of the internets the following sight: Comprehensiveness for the Sake of Truth. In particular, I suggest that you ponder this explanation of and expansion upon the Lambeth Quadrilateral and consider adding your signature to it, as I, and several others whom I respect deeply, have.
If we do, indeed, have a core doctrine, then we must make some effort of defining it. This is as good a start as any. Is this the end of the process? No, but this is a good starting point.