Christianity is nearly-always described as a religion of revelation - in other words it needs to be 'revealed' to us, we need to be told about it; told by - for example - written scripture, handing on by tradition, teachings of church authorities etc.
The contrast is a natural religion that could be reached by each person for himself, by intuition, reasoning, observation etc. based on suggestions from the material that can be found in pretty-much any human culture.
(The paganism of animistic hunter-gatherers is regarded as an example of natural religion; since it seems to have arisen spontaneously - albeit with local variations - all over the world.)
When I became a Christian, it was partly by accepting the need for revelation (and by evaluating the strength of validity in various contending revelations) that I got-out from trying to reach Christianity by my own unaided efforts.
The is no real doubt that mainstream Christianity, in whatever of its denominations or churches, is way too complex to be arrived at except by revelation; but the big question is whether the necessary and sufficient essence is, in principle, attainable without external help...
And, if not accessible during this mortal life, then potentially core Christianity might be accessible after this mortal life (i.e. after death) but before making the choice to accept or reject resurrection.
Over the years I have come around to the idea that Christianity is, or can be, a natural religion; and does not require revelation - indeed, what has traditionally been regarded as revelation may more often be an obstacle than assistance nowadays -- especially since the churches are so corrupted by worldliness (corrupted to the point of value-inversion), and their teachings so vast and (to common sense) incoherent or contradictory.
I think it is largely uncontroversial that theism - specifically the belief in a personal creator God - can be a natural belief. The this world was made, and with some design and purpose, and by a being who is like a person - is a plausible line of reasoning, that often occurs to children.
And the further idea that this God is relates to each of us like a loving parent, is also one that can occur to anyone who thinks about the subject - by analogy with the human family.
In short, all the necessary background to Christianity can be arrived-at without being taught - and (because it is true) potentially confirmed by intuitive conviction.
As for Christianity; I think the core idea is also simple enough, and close enough to the desires of the human heart, that it could be arrived at by almost anyone.
The fear of death and desire for everlasting life is spontaneous; and that this continued life would be embodied is also natural; and that this life-after-life would be in a better world without death or decay are all plausible day dreams - even for a child.
So the necessary materials for a Christian yearning are there. As for the mechanism by which this might be achieved - which is by following Jesus Christ, as a sheep follows the Good Shepherd - this is not something that is likely to be a natural or spontaneous conclusion.
Therefore, natural religion breaks-down at the point where we need to know how to attain resurrected life eternal.
But, since this following of Jesus through-death to life-everlasting happens after our death, it seems reasonable to assume that it is something made clear to us after death.
No matter what a person's experience during mortal life, he could be shown the possibility of resurrection after his death and how to attain it.
Therefore; whether he was born before Jesus's time or lived in a part of the world where Christianity was unknown, or had been taught or reasoned-out some false idea of Christianity - every soul could be given the knowledge of what he need to do to follow Jesus.
Thus, Christianity can be a natural religion. And this fact may turn out to be vital to salvation, as the corruption of the churches continues to spread and advance.