I wonder what people understand, who do not know about christianity?

I recommend that you avoid analogies.
Sagan nailed it

And I recommend that you hand out this recommendation to matter-loving scientists and Saganites first. Typically - predictably and non-scientifically but poetically/analogically - they compare/contrast physical objects and systems with completely other things, as in their rather pathetic "billions of years ago, some star gave its life so that you could live on Planet Earth". Of course, no star has a life to give, nor does it have the capacity for altruistic self-immolation, nor does it have an imagination that would permit it to be compassionately concerned for the existence and welfare of future earth-beings. Another example is when in Cosmos, Sagan favorably compared Hindu religion's "butter-churning" theory of cosmic origins to the modern scientific view, i.e., "the Hindu model, although prescientific, is nevertheless "LIKE" the modern cosmological picture". These figures of speech, and so many others like it, are pure analogy.

Like N.dG. Tyson, Sagan never came close to getting anything right about religion (or the ufo phenomenon, for that matter). On those subjects, he did his science by proclamation - that is, he didn't do science. And he was clueless and uncritical enough to pose himself in front of gorgeous stellar photographs or lush, spreading tree branches and proudly proclaim that, "Like them, I am star-stuff!" But he never posed himself in front of steaming puddles of bloody cancerous diarrhea or victims of flesh-eating bacteria, which are also star-stuff. He salad-barred his cosmos piety in a kind of crude reversal of Genesis's Creator-piety, which also tends to praise all things bright and beautiful, while ignoring the rest.

If there is a core to Christianity, that core is only composed of physical facts

That's just scientism/reductionism. I have already, two or three times now, described Christianity's core as a path into a new life centered in Spirit, a claim you could not or will not bother to refute.

You claim that Christianity's only "real" core is a physical event - Jesus' execution, but that too is a more-than-physical category, for the simple reason that - like his Resurrection - it was experienced in the subjective personhood of the witnesses, which involves private, personal interpretations and subjective evaluations - and of course, memory.

But if you want a physical referent for this, it might equally be said that the experience and the memory of Jesus' death-Resurrection were historical facts as well as neurological facts, because they were stored and interpreted in-and-by the brains of the historical early Christians. Whether or not God actually raised Jesus up, people believed that this had happened, and the correlates of that belief could be tracked in their brain readings - i.e., had fMRI been available back then, surely the physical correlates of the experiencers would reflect the experience and the memory of it.
And of course the only physical proof of all of this would be to discover (good luck) a piece of "the true Cross" or something that Jesus wrote containing his DNA - or to somehow discover and utilize some kind of time-travel technology.

You seem to be claiming that only physical objects, events and processes are true or truthful - ours is possibly the first era to so uncritically confuse fact with truth. And of course, most of us, based on daily subjective feeling and mentation, think that is not the case.

For example, we don't need quantified material data mediated by the senses to realize that we love-and-seek the True, the Good, the Beautiful, we don't need a brain scan to tell us when we're having a migraine, we don't need fMRI to tell us that we know and love-or-dislike a person, an animal, a political group, a sports team, just as we don't require physical objects to understand and manipulate Number and numbers.

Moreover, even the most "objective" scientific experiment or hypothesis originates in the subjective personhood of the experimenter or theoretician, whether as a new way to make money, to satisfy curiosity, to add to the bulk of human knowledge, etc. Science, like religion, exists primarily in the subjective human psyche - neither of them exist in in the external realm of nature.

"Looking outside" through a telescope and confirming that Jupiter has moons - although the instrumentation differs - is as much a subjective experience as "looking within" and finding anything from contentment, lust, anger and insecurity, to one's own Buddha-Nature. The human psyche is the sine qua non of all experience, whether material, social, political, or spiritual.

A poster once opined that in a certain experiment, religious people's brains were scanned while they were having a mystical experience, and that therefore we can now, and at last, see and know what a mystical experience looks like. That's materialism at its crudest. Of course, we are not seeing the mystical experience, but only its neural correlates. The only person seeing the experience is the one having the experience, which is not itself merely a special type of chemical-electrical activity occurring in a four-pound skull organ - which in any case, remains a four-pound skull organ, not a subjective self experiencing God or what have you.

Your comment about spirituality is circular reasoning. You are required to make your case for spirituality (supernaturality) outside the confines of scripture

Oh, but I already did, in my previous post, via the citation from the Apocryphon of John, which you apparently saw fit to ignore, and which challenged your claim that religion is only a co-option of scientific cosmos piety.

In any case, scripture does support my stark, simple claim about transcendent categories and experiences. And so do most other religions and expressions of spirituality; even non-theistic religions insist on the reality of a divine Transcendent which is not always or necessarily a personal Creator-Sky Father, but something like the Tao, the Dharmakaya, the Divine Self, the Atman-Brahman, Buddha-Nature, the Ground of Being, the Sacred, Wakan Tonka, the Absolute, the Holy, the Wholly Other, the Ein Sof, the Pleroma, the Abyss, the Profundity, the Silence, etc.

Even fundamental Theravada Buddhism has its own functional equivalent of deity. As Jesus said things like, "Who sees me sees the Father", so also the Buddha said things like, "Who sees me sees the Dharma", and the following citation supports this:

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… Two meanings [of the word “God”] must be distinguished for its place in Buddhism to be understood. One meaning of God is that of a personal being who created the universe by deliberate design and periodically intervenes in its natural causal processes. Defined in this sense, nirvana is not God. The Buddha did not consider it personal because personality requires definition, which nirvana excludes... If absence of a personal Creator-God is atheism, Buddhism is atheistic.

There is a second meaning of God, however, which (to distinguish it from the first) has been called the Godhead. The idea of personality is not part of this concept, which appears in mystical traditions throughout the world.

When the Buddha declared, 'There is O monks, an Unborn, neither become nor created nor formed. Were there not, there would be no deliverance from the formed, the made, the compounded,' he seemed to be speaking in this tradition. Impressed by similarities between nirvana and the Godhead, Edward Conze has compiled from Buddhist texts a series of attributes that apply to both. We are told that

“Nirvana is permanent, stable, imperishable, immovable, ageless, deathless, unborn, and unbecome, that it is power, bliss and happiness, the secure refuge, the shelter, and the place of unassailable safety; that it is the real Truth and the supreme Reality; that it is the Good, the supreme goal and the one and only consummation of our life, the eternal, hidden and incomprehensible Peace.”

We may conclude with Conze that nirvana is not God defined as a personal creator, but that it stands sufficiently close to the concept of God as Godhead to warrant the linkage in that sense.

(Buddhism: a Concise Introduction. Huston Smith and Philip Novak. Harper, San Francisco: 2003, pp. 53-54)

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The universal transcendent, and the human experience of and union with it, is present in most religions, including Christianity, which shares with them the notion of a new life centered in the Transcendent.