Comparative Religion

What, exactly, is religion? And why does one religious tradition often differ so markedly from another, even when you might not expect it to? Why, for example, are the Abrahamic faiths of Judaism, Christianity, and Islam—despite their common source—often so different? And what kinds of factors separate the beliefs of a Hindu or Buddhist not only from those held by Jews, Christians, or Muslims, or by each other, but also from many who identify themselves as fellow Hindus or Buddhists?