I have to admit that I'm Heathen, however I'll be as unbiased as I can.
How I understand some theories on the development of Christianity starts in Israel with the Pagan Canaanites. There were four notable deities, being El, ruling God of the sky resembling Egyptian Ra, Asherah, fertility goddess and his wife resembling Egyptian Isis or Persian Astarte, Yahweh, war God resembling the wrathful Greek Ares, and Baal, storm God similar to Nordic Thor.
Abraham calls his God "El" or "El elyon", and later his "Elohim" which in the context of the time might be most appropriately translated as "patron God (of a pantheon)" suggesting Abraham was Pagan, honoring many deifies but giving one special attention in exchange for special treatment, as the Athenians were to Athena.
Later there was tension between the temple to Yahweh and the one to Baal. To emphasize Yahweh's status they syncretized him and chieftain El, and competed against Baal's priests. They only were successful by depicting El-Yahweh as a universally powerful God, who was jealous of others.
Full blown monotheism isn't established until Isaiah with "besides me there is no God". Before "God" or "The Lord" refers to an "us" and implies the existence of other divinities, making scholars thing Abrahamic Spirituality was henotheistic, monotheistic to one deity but vaguely agnostic to others.
The Hebrews (coming from Canaanites) were captured by Babylon and Babylonian creation influenced Abrahamic lore, particularly with creation and the Leviathan.
Satan wasn't evil in the lore until much later. "Lucifer" wasn't included in the book until after Roman adoption of Christianity. The story originally is talking about the fall of a mythical king, not an angel.
Satan was interpreted as an evil character in the New Testament at least. Later Christians would liken him to animal-like trickster gods like Pan, Cernunnos, Apophis and Loki. From Loki comes deceptiveness while Pan and Cernunnos give Satan a depiction of wearing hornes or bearing cloven feat. Apophis was the inspiration for the aspects of Satan being a) the serpent of Eden and b) the dragon of revelation.
Jesus was born at a vague time AD. He lead a cultural rebellion against Rome, attempting to make himself king by claiming royal blood from metaphorically or literally descending from Yahweh, as Caesar claimed do with Venus or Ragnar Lothbrok with Odin. "Son of God" was a title having nothing to do with divine genes, but everything to do with the right to be king of a land. Because he wanted to usher in The Israelite Kingdom (aka kicking Rome out), he was committing treason and was punished like all Roman trators and political rebels, through crucifixion. The Jewish Aristocracy only notified Rome of Jesus' movement, so Rome killed him, not the Jews.
The Prophecy of a Messiah of the Jews was understood to be written about someone to establish the Jewish kingdom, independent of Rome.
He was crucified for his political agenda but something unknown happened to compel his followers to think his message was spiritual and not political. This something is often assumed to be the alleged rebirth of Jesus.
Christianity's spirituality started resembling Buddhism and Mystic Judaism. In converting the local pagans of Greece and Egypt, Jesus as a dying God was given stories of native dying God archetypes like Horus and Osiris in Egypt (Osiris died and rose again and was called "King of kings, lord of lords" in the Egyptian book of the dead while Horus, born of virgin Isis Meri/Mary, performed weather miracles like calming storms and walking on water) Dionysus in Greece (died and rose, virgin born, miraculous with wine even turning it out from water, born in a cave or stable, started as mortal but rose into Olympianhood) Mithra in Iran and Rome (born of a virgin, called "the light/truth" worshipped on Sunday) Woden in Germany (died on a tree for the sake of humankind and arose again) etc. this suggests a syncretic attitude early Christians had with paganism, meaning they looked at Pagan mythic characters as being related to, maybe as different forms of, their familiar biblical ones, called "Interpretatio Christiana".
After Christianity was made official roman religion, Rome's colonialist attitude was intensified. The Christian Rome sought the conversion of Germanic and Celtic Europe first.
Already the winter solstice was a celebration from Saturnalia, but celebrating Christ rather than Saturn. The same day was called "Yule" or "Jol" to the Vikings, Goths and Anglo Saxons.
After a genocide on Germanic Royalty, destroying allegedly Odinic, Ingvic, Thorish and other bloodlines, England was easy to subdue into Christian power. Missionaries went into there (pre-conversion) as well as Scandinavia, Germany and the minor Celtic Ireland.
From the Heathens we get Christmas symbology, being Santa Claus, Holly, Evergreen Trees and Gold (from Odin, Mistletoe, Pine and Baldr) and the same with Easter, being eggs, rabbits and spring colors from Goddess Eostre's symbolism as well as Halloween from Celtic Samhain and Heathen Autumnal Festivals (whence comes the association with death, ghosts, demons and magic), called "All Hallows' Day" on Nov 1st, the day before being "All Hallow's Eve" shortened to "Halloween".
The story of Adam, Eve, a tree and a serpent bears suspicious similarity between the Viking world tree and world serpent, especially in the myth of Ragnarok which has a lot of parallels with Revelation:
Jesus-Baldr
Fenrir- the antichrist
Loki/ Surt- Satan/ the dragon
Jormungandr- The Beast
Both also have worldwide chaos by fire, trumpet blowing, darkening of the sun and an ending involving two giants or humans hiding in the tree, implying the ending of Norse Pagam Lore is the beginning of Biblical Scripture.
The Flood Archetypal myth is also repeating in Paganisms, most notably Greek, Egyptian, Middke Eastern and Scandinavian Polytheisms, involving a global flood and two or a few people to repopulate the human species.
The Mediterranean Biblical God was depicted similarly to Jupiter and Zeus, being interpreted as a just, celestial father even carrying the sky-ish and lightning-ish symbolism like the chief European sky God.
The New Christians believed very quickly the lies of the church. Recently pagan Saxons wrote Beowulf, a hero who fights a demon, his mother, a lake of serpents and a dragon, all understood as representing Heathens and Heathenism.
The word for "pagan" used by Christians has changed. Having first arrived in Greece, and the only interactions with paganism having been indigenously Hellenic, the word for "Greek" (Hellenes) was used to describes polytheists. Later in Rome, with it spreading there, the religion had been popularized in urban areas, and the country folk (called "paganī") were left unchecked to follow the older pagan ways. The word for "country dweller" came to mean instead "polytheist",carrying over to English as the word "Pagan".
The Cross comes from a pagan solar symbol of the sun wheel or sun cross. It has variances that look like what we would recognize as swatzikas, crosses and eight armed star like crosses.
"Heathen" is just and English translation of the word.
Hellás- Greece- adj Hellen
Pagus- countryside- adj pagānus- pagan
Hæthe- Eng. Heath- adj hæthen- Heathen
The shifts in this pagan meaning suggests Christians mostly interacted with polytheists firstly as Greek Polytheists, then Roman rustic folk, then as Anglo Saxon Heath folk.
The words "Heathen" and "pagan" then were more popular to use to describe any polytheist like the Vikings and Druids.
As Europeans came in America and Africa, the natives' ways (West African Voodoon and Native American shamanisms) syncretized with Catholicism turning into ideologies hailing saints or spirits who serve a great deity (Yoruba "God" or "Voodoo" and Native American "Creator", "Great Spirit" or "Sky Being").
Paganism is commonly ancestral, so the new converts had a preference to honor the characters from Biblical Mythology as their ancestors, making the depictions of say Abraham, Moses or Jesus as ethnically similar to the dominant race of the worshippers in Christian art.
To Sum it up, Canaanite Paganism combined the sky God and war God to become a monotheistic divinity of the Middle East. Yeshua ("Joshua" or Jesus) seemed to want to fulfill the prophecy of Israel's political liberation, and later was interpreted as an avatar of the monotheistic God. Pagan ideas and symbols from Babylon, Egypt, Greece, Rome and Germanic Europe influenced European based Christianity while West African animism influenced Voodoo, Santeria and the like, lastly Native American shamanic practice influenced Christian movements of the Cherokee, Comandchee, etc nations.
I don't think that the similarities between Christianity and Pagan Polytheisms need to necessarily devalue anything. Christianity is going to have to change tho with these parallels being more well known, but only has to change but not be considered less authentic.
Everything cultural, linguistic and spiritual came from something else. Heathenry, my own community, honors spirits and gods with the Germanic Culture in mind, evolved from a much more ancient Northern European religion with very different names, symbols, values and stories, which came from a much older European-mother culture that too was quite different on the surface. This worldview probably was also just a reinterpretation of an older polytheistic worldview.
I think to satisfy the intellectuality of the religious, the mindset of loose and abstract mythological interpretations and culturally liberal approaches are going to be the future of western spirituality, accepting that religion is metaphorical, subjective and pertains to the spirit and morality, but outside of that it has little practical application.
(I'm using my culture because I know it best) Heathens would tell you that although the lore says the Sun is Sol, a giantess, running away from a sky wolf around the earth or that Odin is a one eyed sage with a flying horse, that doesn't mean we need to accept this literally and become geocentric and looking up for Sleipnir. I'll tell you some are applicable very well in meditation and cultural matters, but have little use in literal interpretation. Although the lore says creation was consisted of a mythic giant who battled with Odin and whose body was used to made the world, I don't think this is scientifically relevant and I accept theories like evolution and cosmological Big Bang as valid.
So Christians I feel should accept their ideologies pagan roots, heritage and past, and, if they so chose, interpret the biblical scriptures with note of cultural biases at the time and the same people's scientific ignorance, leaving the interpretations abstract and strictly spiritual. I see these characteristics not only in Paganisms but Liberal Christian Movements and the Unitarian Universalist Movement.