what is religion and What happen to the roman religion?
These are two separate questions, the first of which is too big to answer fully here (it would require libraries!), and the second really requires clarification, which I will attempt. Religion is essentially the belief in a supernatural power or powers. In most forms, it includes a story of the creation of the universe and a description of our human interactions with that power. It usually includes a set of beliefs and values relevant to the supernatural power, although there is tremendous variation possible within these. The supernatural power may be personal and individualized (as with the “Religions of the Book”: Judaism, Christianity and Islam), but (as in some esoteric forms of Buddhism) need not be. The supernatural power may take a direct and personal interest in the believer, and may be benevolent or not. There may be multiple powers– either dualistic, frequently balancing good with evil (Zoroastrianism, for example), or communal as with the Greek or Norse gods, who at times t