The three major monotheistic faiths of the world (Judaism, Christianity and Islam) have clear, historical ties to each other; particular, Christianity sees itself as a "superior" successor to Judaism, while Islam sees itself as "superior" to both Christianity and Judaism.
Judaism is the oldest of the three, with its roots in the stories of Moses, the exile from Egypt, the visions and revelation at Mount Sinai, and so on. The God of the Jews is therefore a product of the circumstances of its adherents; a God of the desert that wreaks righteous destruction on the polytheistic Egyptians, for example.
But what of Judaism's successors?
Zeus, or the Sun-God?
Christianity's religious symbolism is a natural by-product of the culture and circumstances of the day. It has been well-documented (and easy to find on the internet) that much of Christian symbolism stems from a combination of Egyptian, Roman and Greek influences.
For example, the importance of Christ's birthday coinciding with the winter solstice, and the coming of the "three kings". This fits in with an ancient Egyptian fable of the importance of Orion during the winter solstice: Orion's belt at its lowest point is level with the horizon at this time of year in that time of year. The "three kings" thus arrived from heaven, after spending the rest of the year in the stellar plane. The belief in Christ as the "son of God" also has parallels in Egyptian religious symbolism, as well as in other pagan Middle Eastern religions.
Early Christianity was an underground religion in the Roman Empire for its first few centuries. There were no "churches" as we know them today; adherents either used caves or catacombs as impromptu places to worship their faith, with only richer people of faith building converted chapels in rooms of their villas. Interestingly, when Christ was portrayed in artistic form in these early years, he was usually shown to have an uncanny likeness to Alexander The Great; with curly hair, head tipped to one side, with a smooth-skinned (almost child-like) face and carrying a staff. These were his "hallmarks". An alternative version is that he was seen as an alternative to Apollo, the Sun-God, and that the symbolism of worshiping the "Sun-God" as the primary source of religious power on earth. In those early centuries, many Christian converts were still hedging their bets, and it comes as no surprise that Christians saw the need to drape their faith in familiar pagan symbolism that wooed the waverers (more on that tactic later).
Christianity became the official faith of the Roman Empire with the conversion of the Emperor Constantine on his death bed in the middle 4th century. However, prior to that he made public his advocacy of Christianity throughout the empire, if not publicly declaring his faith. His moment of "revelation" came in battle, when he was fighting in a civil war to secure his position, at the battle of Milvian Bridge. Here he saw the "light of God" in the sky, seeing it as a sign that God was on his side. Still, he hedged his bets, using the battle to declare "Sol Invictus" (the undefeated Sun) as his official motto. The worship of the Sun as God-figure is found in a multitude of pagan belief systems (such as "the Logos" of the Greek philosophers), historically borrowed by Christianity.
During his reign as emperor, one of his most important decisions was to make churches public buildings, no longer hidden from view. Here, Constantine borrowed from existing Roman structural design. Roman magistrates' courts were known as "basilicas"; Constantine simply copied the design, but moved the entrance from the middle of the longer wall, to the middle of one of the end walls, thus changing the perspective for the building's purpose to emphasize its length.
Finally, Constantine presided over the changing of the understood face of Christ; from that resemblance to Alexander/Apollo, to the bearded face of Zeus/Jupiter. It is this image that has stood (barring cultural modifications) ever since.
There are plenty more pagan links with Christianity, but for the sake of brevity, this will suffice. Now, we can look at Christianity's "successor"...
The cube in the desert
Islam's holiest site, which every Muslim prays in the direction of, is the "Kaaba" ("Cube" in Arabic). What is it, and what does it represent?
According to Islam, the Kaaba was a cube-shaped stone structure built by Abraham as a temple to God (Allah). But by Mohammed's time, it had become a temple for the many gods that the local tribes worshiped (as many as there were days in the year). Mohammed's role was then to restore the Kaaba to its original purpose; so the temple was cleared of the many idols, and had a mosque built around it. Centuries later, after many modifications and extensions of the mosque (and rebuilding of the Kaaba after floods), this is the structure that Muslims pray towards. According to Islam, the Kaaba is older the the temple in Jerusalem, and thus the oldest temple devoted to God (Allah).
That is the official version; the evidence shows something very different.
For Mohammed's version of events to be true (i.e. that the Kaaba is older the the Temple Of Solomon in Jerusalem), the Kaaba would have had to have been built around two thousand years before Christ. However, (as we see here), Mohammed himself seems to contradict this point. Besides, respected authorities on the topic (see the previous link) seem clear the the Kaaba was a relatively contemporary building in Mecca at the time of Mohammed's birth.
The fact that it is still standing at all (and wasn't destroyed completely as a pagan temple) was probably due to an act of compromise to cultural sensitivities by Mohammed. There is also evidence that there were other "Kaabas" in the Arabian peninsula at the time of Mohammed, albeit not made of stone, but wood, for example.
Another important feature of the Kaaba is the "black stone", embedded in one of the outside corners of the wall. This is recognised as a fragment of a meteorite that descended to earth in ancient times, close to Mecca. Of all the idols existing in the Kaaba before Islam, this one alone was retained by Mohammed. Pre-Islamic Arabs had a tradition of kissing the Black Stone; this "pagan" tradition has continued in Islam, and is an important feature of the "Hajj".
The pagan history of the Kaaba, and the meteorite fragment revered in its wall, seem to jar badly with Islam as a religion so seemingly hostile to the concept of idolatry. And yet Muslims today emphasize that they are not praying to the "Kaaba" itself, let alone to the meteorite fragment within its walls, but instead use the building a worldwide point of reference that all Muslims can relate to, uniting them.
Apart from the Kaaba, the symbolism of Islam also features many other parallels to other (pagan) faiths. The major deity that the Pre-Islamic Arabs worshiped was the Moon-God, Hubal. This was a deity whose idol had a human form (probably from red agate), but whose right hand was gold. Seven arrows were used for the purposes of divination before the idol during rituals.
As anyone knows, the crescent moon is integral to the symbolism of Islam. Every mosque in Islam has a crescent moon at its highest point; the crescent moon is the most famous symbol of Islam. Does this mean that the "Moon-God" was replaced by "Allah"?
Muslims vehemently refute this, and it is clear that the use of the moon as symbol was probably not a direct form of replacing Hubal with "Allah"; in battle at the battle of Badr, his enemy Abu Sufyan evoked Hubal with the words: "O Hubal, be high", to which the Prophet replied: "Allah is greater". Is this where Mohammed also got the inspiration for Islam's most famous verse?
From this we can make an educated guess that the use of the moon as the primary symbol in Islam may have been partially to assuage local Arab pagans with some familiar imagery, while yet claiming that "Allah" is higher than Hubal.
Furthermore, the use of the moon is also an essential part of a Muslim's practical life: it is needed to calculate the correct time to pray (more on that in a moment). It was for this very practical reason that the early Muslims became such expert astronomers compared to their contemporaries.
Other aspects of Islamic rituals also have parallels to other faiths.
The importance of praying five times a day (and ablution beforehand) predates Islam. Zoroastrianism uses this ritual as part of its sun-worship (though they pray in the direction of the sun, wherever it is in the sky at the time). However, in Mohammed's day, the Pre-Islamic Arabs would pray in the direction of Mecca.
An essential part of the "hajj" is the seven-times circumambulation around the Kaaba. Again, this ritual of passing seven times around an object of veneration predated Islam. Pre-Islamic Arabs did the same around the Kaaba, only in order to please Hubal instead. This ritual is also practiced in Hinduism; a faith older than Islam. In the Hindu marriage rite of "Satphere", the couple pass seven times around a fire, where religious phrases are recited, the same concept as in Islam.
There are other rituals, such as that of "Ihram", which is another Pre-Islamic ritual involving washing as wearing "Ihram" clothes to enter Mecca. A final example is the seven-times walk between Safa and Marwa mountains close to Mecca, which was another Pre-Islamic ritual retained by Mohammed for the "hajj".
So as we can see, both Christianity and Islam as replete with pagan symbolism.
It is no surprise that both faiths would have needed to adapt to the social and cultural rituals and circumstances of the time in order to flourish; it allowed waverers of the time to have their cake and eat it.
The irony here is that Christianity chooses the symbolism of the sun as its religious banner; Islam, on the other hand, sides with the moon. No wonder the two don't get on.
The sad truth is that while the pagan origins of Christianity can be debated in Christian societies without seriously expecting to receive physical harm or threats (except perhaps for parts of the Evangelical USA, or Africa), the same cannot be said of large elements of Muslim society. This is something that Muslims have to an extent brought onto themselves, by moderate (and progressive) Muslims refusing to tackle the problem of the growth of Radical Islam in the last thirty years.
Radical Islam sees any debate within the faith (such as of the pagan origins of some Islamic rituals) to be worthy of a death sentence, or a severe punishment at best; go to Pakistan, for example, and see what happens, if you try to discuss the pagan origin of some key Islamic rituals. Only a very brave (or foolhardy) soul would do so. It is no wonder that Islam has such a poor image with the rest of humanity.
Herein lies the problem with many aspects of religion; it is immune to debate.