Is Christianity dualistic?

@Anonymus

You unfortunately proofed it as dualistic. Because you say it's only heaven or hell. Here is how dualism is defined:

"the division of something conceptually into two opposed or contrasted aspects, or the state of being so divided. "

Platonist/Neoplatonist dualism is the idea that the mind or soul is the divine part of us struggling toward liberation from its bodily imprisonment. Dualism holds that the soul is the real self, by nature eternal and pure. Plotinus (3rd century A.D.) saw the human being as a microcosm - a small-scale version of the cosmos which has 3 ranks - from the unknowable One, to Mind, to matter. So there are 3 ranks of people, he said; the pneuatikoi (spirituals), the psychikoi (soulish) and the sakikoi (fleshly). This anthropology, known as trichotomy, is a minority view in some Christian circles today. It was defended by C.I. Scofield in the 'Scofield Reference Bible' of 1909, and in the 1960s by a few others.

Mainstream Christianity resists trichotomy. You can read an in-depth response to this in "The Christian Faith" by Michael Horton, pp 374-75. But dualism also requires knowing what Rene Descartes (1596-1650) said about it as this influences modern stances. Some orthodox Chrisians have reacted against modern dualism and advocate a modified monism, which emphasizes the psychosomatc (body-soul) unity of human beings. Not all of them are sure there is any immaterial soul. Some claim that the soul is not distinct from the body, while rejecting the naturalistic reduction of the human person to material processes.

Criticism of dualism in general and a radical soul-body dualism in particular is a recurring motif in Reformed theology up to the present day. To grasp the issues, you need to understand these key distinctions: Radical dualism contrasts a divine soul with its bodily prison house (as in Plato). In Descarte's version, there is no interaction between mind and matter. Monism is equally radical in denying any distinction between body and soul. Biblical duality-in-unity recognizes a distinction without denying the unity of the human person. Scripture provides an entirely different map for understanding the human person. The Bible is clear that the great ontological distinction is between God and everything else, not between spirit (or mind) and body (or matter). We do not have a body (as if the soul were our real selves); we are created as a psychosomatic (soul-body) whole, as persons. Our bodies (including our brain) and souls are not separate compartments, but interactive aspects of our personal existence and activity. That is the mainstream Christian view. And, at bodily death, there is a temporary separation of body and soul, the soul having a living presence before God at bodily death (e.g. Ecclesiastes ch. 12). There will be a reuniting on the Day of Resurrection, so the intermediate state is not the final self. The 'real self' is the whole self.

Originally, it is not.  Unfortunately, how Christianity is typically taken, in recent times, it is; especially, how people often depict God as "elsewhere", in some distant place and time.  How is it possible to be "separate and apart" from an 'Omnipresent God' expressed around, in and through, 'Everything', 'Everywhere' at 'All Times'?  The God Christ spoke of is described as a Transcendent and Immanent Reality.

Actually it's markedly non-dualist.  For instance, it sees human consciousness as necessarily embodied.

All religions are "Y-n and Yang"  ... Since always! 

https://en.wikipedia.org/wiki/Yin_and_yang

No.  Here is what you need to believe:  Death leads to heaven or hell.  The Lord Jesus Christ is God, who died on the cross and shed His blood to pay for all of our sins in full, and then Jesus was buried, and then Jesus resurrected from the dead.  Nothing you could do will ever pay for any of your sins.  Jesus already paid for all of our sins with His death on the cross and His shed blood, and then Jesus resurrected from the dead.  So the only way to get into heaven and avoid hell, is by believing in the Lord Jesus Christ, without adding any of your own works.  See Romans 4:5.  It is too late for you to be saved, after death.