The early Christians knew the idea of reincarnation, but I do not know how they understood the concept: as a superior essence lasting on, or an individual soul moving on from body to body. There are religion-scientists and historians who imply that maybe Jesus, before he reached the age at which the bible starts to tell tories of him, maybe was travelling to India and came into contact with Buddhist concepts. That may very well be possible, since it would explain why there are so many parallels between Buddhist and Jesus' ideas (and I mean Jesus, not what the church made of him, between the church and buddhist view of the world obviously there is no parallel at all).
Buddha denied the idea of an individual "soul" that survives the body and begins a new cycle once the former body has been destroyed. Man is not a host and soul is not a Goa'uld worm.
However, buddha hinted at the existence of an existence that must be understood in a higher context, a true self that in basic is a One-ness, doesn't get born and thus cannot die, and of whose existence all forms are just a temporary reflections interacting with each other and giving birth to a dance of colours and shadows, each such "reflection" of the higher Oneness not so much being just a part of it but being "it" in completeness, though no reflection has any substance and existence in itself. These reflections, that we call the world, the living beings, the things, are empty in themselves, they show us a world that the way we perceive it is just a delusion - that delusion exist, we see it and we fall for it, but what it shows us in content, is not real, and has no substance. It's like a fata morgana.
In the end, if it is something like this, we all must not find any spiritual fullfillment or justification for our existence, for since we are already "there" (since we, the reflections that we are, are the One-ness anyway), we must not and cannot go anywhere anyway. We can just trouble the water by shaking the waves without need, believing in the false idea that we must "reach" something and must try to get into a "heaven" because we want to avoid a "hell". Heaven and hell are two states of human mind that man forms up - all by himself, and there is nobody and nothing promising him reward or threatening him penalty for doing so. We can trouble the water and add to the dance of reflections we call "the world", if we want. But we could as well let it be.
This is my understanding of "sin": to lose or to reject this knowledge about our already present, always existent "higher" origin, and to start making things comolicated and worse by trying to acchieve a solvation that we already are embedded in, and never had left: the salvation of understanding who, or better: what we are, and what we are not. Sin is - lacking insight, lacking knowledge, lacking own experience. We mess up things by your egos' narcissim, and our intellect running amok since we do not keep it under control. Our clever ideas and fantastic conceptions run an eons-long olympic competetion of who can run the fastest, jumps the widest, reaches the highest. Our egos claim medals for out acchievements in this championship, and it makes us believe that once we have enough medals, we will be saved and will be given access to a paradise, "paradise" understood materialistically or religiously. Not only relgions are prone to falling for this trap - scientists and social reformers can be that prone, too. The result can be unjutsified, uncritical optimism into materialistic ideas and concepts, from the hedonism of the capitalistic world to the uncritical implementation of possibly dangerous technologies that do not get crticially questioned because they are new, and "new" makes them attractive. Where all this happens at the cost of our exploration of whom and what we really are, then nothing good usually comes from it, that way we mess up the world we live in and harm ourselves in the best of intentions. It's just that these our intentions maybe are reaching too short.
Spirituality in my understanding is trying to understand the nature and reason of our existence by introspection, by observing how our minds call the world that we believe to perceive into existence, and how "mind" manifestates itself. This is the path of experiencing ourselves. Science tries to make conclusions on the nature of reality and things existing by describing them empirically, checking for patterns that may reveal to us why and how things are, where they come from, and where they go. Spirituality is about going into the thing itself, science is about describing the thing from the outside as best as is possible, and from the outer appearance make conclusions on the inner reason. Objectivity and a lack of sentimentalities are virtues in both approaches.
Religion is neither the one, nor the other, it is hallucinating and fantasy. It obstructs the path of introspection and own experience by raising a dogma that should neither be examined nor questioned, but simply should be believed and taken for granted although there is no objective justification for doing so, it just promises to be a shortcut of greater comfort and easiness, to bypass the more difficult path of spirituality and/or science. You are spiritual for your own well-being and by changing yourself becoming of benefit for others as well. But you are being talked into being religious not for your own well-being, but the interest of others for gaining control and power over people, inclduing yourself. Spirituality and (institutional) religion (dogmas) are antagonists, seen that way.
We all are dreams within one dream. Dust and shadows, winds in the leaves, the waves on the ocean's surface. Stick to the things in life as if they are substantial and real, and you will become a prisoner and miss the meaning of it all, being blind and fall to despair over the existential questions of life. Let it all go (even your desire to let things go
), and become free. We shall deal with the things of life as if we do not own them, not craving for either poverty nor wealth, neither desiring them nor refusing them, but taking things for what they are: having no substance in themselves, being mere reflections of what lies behind, sometimes, rarely, shining through between the lines of reality, although it is always there.
A later teacher of mine, running a taoistic-buddhist centre in Germany, once wrote in a book this (tranlsated from the German):
The letting go of all ideas of God and all religious thoughts one is fond of, is an absolute prerequisite for true mystical experience. […] But experience has shown that the letting go of personal idols and religious symbols is espe-cially difficult for those, whose personality structure shows the strongest egocentricity and focussing on themselves. They are afraid to lose everything, and therefore they cling to their small, mortal self with all their might. When one is looking closer to it, one will recognize that most people are not about a living experience of the divine essence, but are more about a maintaining of their personal ideas of the god they are fond of, and about wallowing religious feelings. But true mystic has nothing to do with emotional rapture and inappropriate holiness, these belong to the realm of mysticism, which only is a distortion of true and pure mystic. […] Man in general tends to fooling himself and looking for a short-cut, a religion of superficial consolation, an ideal world without problems and challenges, where everything falls into its’ correct place… […] The clinging to superficial forms and religious practices is one of the greatest dangers on the spiritual way. They are shackles which bind us to signs and symbols which actually should only show us the way inwards. Therefore every symbol shows towards something that is beyond itself and that cannot be named or displayed. To go beyond religious signs and symbols therefore does not mean to refuse these symbols, but to strive for what they are pointing at.