“Every invasion or collision or mingling of races thus brought their respective religions into contact and rivalry; and as no priesthood has been known to consent peaceably to its own downfall and the degradation of its own deities, we need not wonder that there have been perpetual wars for religious ascendency. It is not unusual to hear sects among ourselves accusing each other of idolatry. In earlier times the rule was for each religion to denounce its opponent’s gods as devils” – Moncure Daniel Conway
All our gods and demons are mutts. Sure, they descend from world-conquering wolves that roamed the earth in search of followers, but our human propensity for conflict with the neighbors had the consequence of domesticating them, cross-breeding them and turning our mythological critters into mongrels of dubious parentage. If you have a particular theological ax to grind – maybe you’re a big fan of monotheism or a devout Pastafarian adherent of the Flying Spaghetti Monster – such a notion might be a little off-putting, and most folks look at the litany of dead gods over the past few millennia, and in an effort to be vaguely inoffensive, suggest that either (1) my god can eat your god, (2) there are obviously no gods, or more charitably and ecumenically, (3) there is some sort of godhead, and we all just see different aspects. No shame in playing the odds and making Pascal’s Wager (“it is in one’s own best interest to behave as if God exists, since the possibility of eternal punishment in hell outweighs any advantage of believing otherwise”), given the general absurdity of the universe. This of course suggests that the upside to being a god is the whole immortality thing, but the downside is over time, it’s virtually guaranteed that your brand will get watered down, and you just might end up as somebody else’s demon. Theology is after all, extremely crowded with moribund gods.
The hell of dead gods is as crowded as the Presbyterian hell for babies. Damona is there, and Esus, and Drunemeton, and Silvana, and Dervones, and Adsalluta, and Deva, and Belisama, and Axona, and Vintios, and Taranuous, and Sulis, and Cocidius, and Adsmerius, and Dumiatis, and Caletos, and Moccus, and Ollovidius, and Albiorix, and Leucitius, and Vitucadrus, and Ogmios, and Uxellimus, and Borvo, and Grannos, and Mogons. All mighty gods in their day, worshiped by millions, full of demands and impositions, able to bind and loose–all gods of the first class, not dilettanti. Men labored for generations to build vast temples to them–temples with stones as large as hay-wagons. The business of interpreting their whims occupied thousands of priests, wizards, archdeacons, evangelists, haruspices, bishops, archbishops. To doubt them was to die, usually at the stake. Armies took to the field to defend them against infidels: Villages were burned, women and children were butchered, cattle were driven off. Yet in the end they all withered and died, and today there is none so poor to do them reverence. Worse, the very tombs in which they lie are lost, and so even a respectful stranger is debarred from paying them the slightest and politest homage (H.L. Mencken, Memorial Service).
Luckily, the celestial afterlife is eternal, so even if you’ve lost your street cred and nobody bothers to build you ziggurats anymore, your memory will live on in somebody else’s narrative. Sure you’ve been demoted, but the pension plan is none too shabby. Once we decided to start wondering what was going on over the next hill and rubbed elbows with our unsavory neighbors, we realized that we all don’t worship the same god or gods. When you inevitably go about conquering the guy next door, it poses a serious theological problem. Sure you sacked the city, pilfered the treasury, and have the former chief executive’s head on a pike, but now you have to get down to the messy business of ensuring that the crops are harvested, the taxes are paid, and the common man isn’t plotting your assassination in the back rooms of the agora. You can neither slay the old gods, which would provoke popular resistance, nor can you express disbelief in the very existence of the old gods, since that suggests that some invisible friends are less invisible than others, leading to a skeptical spirit that endangers belief in the new gods. You need yourself a Korean Taco, a fusion of religious cuisines that is palatable to everyone in that it (1) validates the existence of the old gods, either as demons or mistaken impressions of the new gods, or (2) a recognition of the old gods, not as gods, but as nasty demons that have been tricking you all this time. This has been the pattern throughout history, and no religion exists today that in some way has not preserved the traditions of the conceptual conflicts from which it was born.
Before you start looking for kindling and setting up the stake to burn me on, consider the case of Zoroastrianism. Hinduism and Zoroastrianism seem to share a Proto-Indo-Iranian origin, a common inheritance of concepts including the universal force rta (Sanskrit rta, Avestan asha), the sacred plant and drink sauma (Sanskrit Soma, Avestan Haoma) and gods of social order such as mitra (Sanskrit Mitra, Avestan and Old Persian Mithra) and bhaga (Sanskrit Bhaga, Avestan and Old Persian Baga). At some point, migrations resulted in distinct Indo-Aryan and Iranic cultures. Yet, there are a lot of shared elements in Zoroastrianism and historical Vedic religion, with a cultural break occurring somewhere around the 10th Century B.C. So, you wind up with a lot of similar terms that take on opposing meanings. Those who were gods in the Zoroastrian tradition get demoted to demons in Hindu pantheons.
The operation of this force in the degradation of deities, is particularly revealed in the Sacred Books of Persia. In that country the great religions of the East would appear to have contended against each other with especial fury, and their struggles were probably instrumental in causing one or more of the early migrations into Western Europe. The great celestial war between Ormuzd and Ahriman — Light and Darkness — corresponded with a violent theological conflict, one result of which is that the word deva, meaning ‘deity’ to Brahmans, means ‘devil’ to Parsees. The following extract from the Zend-Avesta will serve as an example of the spirit in which the war was waged :—’All your devas are only manifold children of the Evil Mind—and the great one who worships the Saoma of lies and deceits; besides the treacherous acts for which you are notorious throughout the seven regions of the earth. ‘You have invented all the evil which men speak and do, which is indeed pleasant to the Devas, but is devoid of all goodness, and therefore perishes before the insight of the truth of the wise. ‘Thus you defraud men of their good minds and of their immortality by your evil minds—as well through those of the Devas as that of the Evil Spirit—through evil deeds and evil words, whereby the power of liars grows.’ That is to say—Ours is the true god: your god is a devil. The Zoroastrian conversion of deva (deus) into devil does not alone represent the work of this odium theologicum. In the early hymns of India the appellation astras is given to the gods. Asura means a spirit. But in the process of time asura, like daemon, came to have a sinister meaning: the gods were called suras, the demons asuras, and these were said to contend together. But in Persia the asuras—demonised in India—retained their divinity, and gave the name ahura to the supreme deity, Ormuzd (Ahura-mazda). On the other hand, as Mr. Muir supposes, Varenya, applied to evil spirits of darkness in the Zendavesta, is cognate with Varuna (Heaven); and the Vedic Indra, king of the gods—the Sun—is named in the Zoroastrian religion as one of the chief councillors of that Prince of Darkness. But in every country conquered by a new religion, there will always be found some, as we have seen, who will hold on to the old deity under all his changed fortunes (Conway, 1879, p25-26).
Maybe you’re feeling a little comfortable in your monotheistic conceits, happy in the knowledge that your high god has bested all rivals and established himself as the celestial CEO. Before you start feeling too good about yourself, consider that Zoroastrianism had a lot of influence on everything from Second Temple Judaism, to Gnosticism, to Christianity, and Islam, which simply recapitulated the same mechanism of theological accretion and absorption that defines human religion from the moment we started painting funky critters on cave walls. Gods eat other gods. The digestion process varies, but the result is generally the same. They fertilized the theology of the next generation. There is no graveyard of the gods, just a hospice.
References
Conway, Moncure Daniel, 1832-1907. Demonology and Devil-lore. New York: H. Holt and Company, 1879.
EsoterX 
Humanity can’t get ahead of the old deities, let alone the new ones. In the beginning there were many different ethnicities. Each had the prerogative of claiming its own personal god(s). As these ethnicities came under conquest and assimilation, their gods were added to the conqueror’s gods, but in a definitely inferior position. This resulted in an often confusing polytheistic plethora that only a few understood or appreciated. Monotheism went even further by postulating a single unique god, with a multitude of angels to do the mundane tasks – but where did the angels come from? You guessed it – the former gods of particular ethnicities and religions. So, the old gods aren’t really dead – they just faded away and reclothed themselves as various kinds of angels – of course, some chose the demonic option since one man’s angel can be another man’s demon.
The H. L. Mencken quote is interesting, in that it is a fun rhetorical flourish, but (like so many such rhetorical flourishes) is almost entirely incorrect.
Most of the Deities he listed are Romano-British, and none of them would have had more than a few thousand worshippers at most, since many of them only had a regional cultus confined to a relatively small area. None of them had large temples–nothing larger than an average house, if indeed they had that–and that several of the various sacerdotal roles he attributes to them applied to none of them (archdeacons, evangelists, bishops and archbishops in particular), and likewise their overall clergies would have been likewise quite limited, is a further mark against it. And, no one dedicated to those Deities would have even thought burning anyone at the stake was even remotely right or even possible for them, as regulation of belief was not their job, and most certainly no armies defended any of them against the ravages of “infidels” of any sort.
But, it sounds neat, doesn’t it? It mainly shows what religion these days is about, not what religion back then was like.
The dualistic concept of gods and demons runs through all these religions, but I question dualism itself.
Disciplines like astrology, (a “symbolic language” like dreams) although they rely on god-symbolism, seat the soul within the individual, who is self-determining and influenced (although not controlled) by the angles the planets make to each other. The individual is expected to look within to find right action within a range of possibilities. Every sign and every planet contain strengths and weaknesses that are characteristic of that sign. For instance, Leo is strong but sometimes domineering, and finding a balance between the two extremes is part of the growth challenge.