Many textbooks of comparative religion can give us guided tours through Buddhism, Islam or Confucianism. But the mass of information they offer is often dead and irrelevant if we're after a simple way of getting at the religion of the fellow doing Zazen in the next room, the student from India who quotes the Bhagavadgita or even the good guy who says he is a Christian but doesn't fit the part.
Religions are like plants.
There is no end to learning botanical names to classify them. Yet most of us can tell evergreens from those that drop their leaves in the fall. We know ferns, grasses and cacti, and we can name at least roses, tulips and dandelions. It is no more difficult to learn to recognize some basic varieties of religions. For this I suggest a four-point classification system which goes to the heart of the religion in question without your mastering centuries of historical detail.