However, I think it may in the past have broadly been true that "everybody" (i.e. most people) was a Christian. In other words; perhaps it was once possible to take for granted that people would choose to convert to Christianity if they:
1. Knew what Christianity offered - i.e. resurrection, eternal life, Heaven. (i.e. A "better deal" than any other religion.)
2. Believed these claims were true.
Thus early Christian missionaries seemed to emphasize that Christianity offered the believer more than their previous religion - so that the potential convert wanted to believe it was true.
Then the missionary tried to convince the potential convert that the missionary's claims were true.
This 'convincing' was, in the early days, often by miracles, answered prayers, and other 'supernatural' means.
The early days of Mormonism provide a well-documented example of this process, when the first wave off missionaries were sent to England.
To simplify; the missionaries would describe what was 'on offer' from this new version of Christianity (representing, for many people, an enhancement of the trad-mainstream Christian afterlife); and then (to convince of truth) the missionary would give the potential convert a Book of Mormon (having described its miraculous provenance).
This was done with instructions to read with a truth-seeking-intent, and while doing-so to ask God in prayer whether the book was true - thus the potential convert was seeking a personal divine revelation of the validity of the Mormon claims (the availability of such personal revelations being a major element of Mormonism, and the Book of Mormon itself).
This 'method' worked sufficiently well in the middle 1800s that the missionaries would convert a couple of people every day, on average. Most of these converts would be sufficiently convinced and enthused by their new religion to leave their homes and families (forever, potentially); and emigrate thousands of miles over sea and land - to settle permanently in and around Salt Lake City.
Such experiences with Christian conversion fit with the assumption that if only a person knew what Christianity offered, and believed it; then he or she would become a Christian.
But that was then; this is now.
Now it cannot be assumed that people will, in general, want an afterlife at all; and even if they do want an afterlife, that they would desire the kind of afterlife that Christians describe.
(For instance, it seems that many people want something more like 'Nirvana' - a barely-conscious, depersonalized state of timeless bliss.)
So, even if such people could be convinced of the reality of what Christians claimed, and convinced that they themselves could obtain this outcome - they would not want it, but instead something else.
Furthermore, there are many modern people cannot be convinced of the truth of Christianity; and the reason they cannot be convinced is not due to lack of evidence, but because their fundamental assumptions (i.e. metaphysical assumptions) rule out the possibility.
In other words, they do not believe that miracles are real, or that answered prayers could happen. Any possible evidence is already explained-away by prior exclusion.
An apparent miracle Must Be due to delusion or fraud; an answered prayer just 'wishful thinking' operating on coincidence etc.
An excellent example of this mind-set is depicted by CS Lewis in The Last Battle, the final book of the Narnia Chronicles. There are a group of dwarfs who are actually on the threshold of Heaven, surrounded by Good Things - but who have decided that the whole set-up is a malicious trick.
The dwarfs therefore actually perceive almost everything Good as trash: good food tastes of muck. Any pleasing event is a deception.
Their whole mind-set is focused on a determination not to be fooled - and therefore a determination that every-possible-thing is (underneath the surface) malign and nasty.
Such is an accurate allegory of many modern people.
- They (we) have pre-decided that this is a purposeless, meaningless and impersonal universe.
- Any suggestion that there is a loving God who created it, is regarded as childish delusion.
- Such suggestions met with incredulity, and regarded as an attempt at manipulation.
- Any 'evidence' of a miraculous or supernatural kind, is explained-away as necessarily fraudulent.
In conclusion; however differently it was in the past; mainstream modern people are Not latent Christians; but indeed are pre-immunized against serious belief in religion generally - but Christianity in particular.