My question is, is will decide that who is muslim and who is not...?
The gentleman worked with the Hashoo group..
That decision is up to god. However, there are clear guidelines on how to meet that criteria in a book that is unaltered since its presence on earth. That book has been the source of conflict as well, but there are certain clear modalities that 95% of Muslims agree on and they agree on it in good theological foundation. Hence, while the last decision may be left to god.. one stance must be taken rather then focusing on ambiguity for the sake of ambiguity.