QUEST EONS
There is archaeological evidence to suggest that the sun god motif endured at least as long as the judeo-christian gods (Yawheh, Elohim, God, El, Jesus, Holy Ghost, etc.) have so far. There is evidence of a sun-god in Egypt going as far back as 3000 BCE with Atum, later to be called Re. The pre-dynastic Egyptians worshipped animal deities and, later, goddess figures were found in Egypt and other places where neolithic peoples were developing agriculture (around 7000 BCE).
The goddess figure seems to have endured the longest and appears consistent in its image as a voluptous, (perhaps well-fed) female figure. One found in Catalhouyouk in Turkey is of a rather large woman seated with her hands on the heads of two large feline animals. This motif appears in many cultures of the period in many geographic locations.
Christianity is one of the youngest religions that are in existance today, and Hinduism is probably the oldest of all surviving religions.
I think the endurance of christianity today is attributable to: 1) the evolution of religious thought in humanity (see Robert Bellah, 1964) through stages of development that have allowed a lot of trial and error to determine what works to create and maintain an organizational heirarchy; and 2) the advent of writing, which allowed the early jewish authors and later christian authors of christian texts to establish doctrine and "write out" past religious thoughts and beliefs. The Canaanite pantheon of gods was effectively adopted where possible and excluded from doctrine where not during the invention of Judaism.
Maintaining the hierarchy is of great importance to maintaining power and wealth and the exclusion of past religious norms, such as prophets and shamanism is controlled by the current priests and officials as christianity is established and maintained. Prophets are a concept of the past (there haven't been any universally accepted prophets in Christianity for over 2k years) and shamans are considered heretics, unholy, works of the devil, heathens, etc.
But there are many things about the Greek religion that survives through christanity (you mentioned this specifically), such as angels. Look at the representations of nikes that are on
friezes and
statuary and date easily as far back as the 6th century BCE (though I think that the two images I linked date to around the 3rd century).
I, personally, think Christianity will disappear. Its downfall will be its own reluctance to progress with the speed needed to keep up with the rest of society. Already, heated debates rage over same-sex marriage, creationist dogma, and other silly points that the fundamentalists of the cult have chosen to be their last lines of defense. Ironically, it'll be these things that cause the downfall of their cult. That's just my opinion, but one I base on the observerance of other cults in humanity's past and their downfall.
Even Hinduism, the oldest of the world's major religions, is doomed to the same fate as technology and societal advancements progress in countries where it is practiced.
Islam, I think, will survive them all, but only because the countries where it is practiced most seem to stay in conflict, which is effective in obstructing societal progress. Though, I've seen some indications that there is a young generation of atheists growing in Iran, so who knows?