When I became a Christian, I accepted that there were natural and revealed religions: and Christianity was a revealed religion. It was an 'historical' religion; which made claims about human history and 'therefore' (it was said) some history must be taught, learned and accepted.
A natural religion like the animism of hunter gatherers was the natural and spontaneous spirituality, and paganism was an elaboration and formalisation of this spontaneous animism (totemic religions being a half-way house). Hinduism was perhaps the highest development of this spontaneous paganism - yes, it is full of culturally specific detail, but something like the polytheism of Hinduism would form in all civilizations (eg Ancient Egypt, Greece and Rome), by culturally inflected spontaneous developments...
But, supposedly, Christianity was not like this; it was (like Judaism, like Islam) one of the revealed religions.
In other words, the idea was that we could Not work-out Christianity for ourselves, from our spontaneous inner feelings and reactions, and natural ways of thinking about the human condition and the world in general. We needed to be told about Christianity; or brought-up in it.
The idea was that - if a bunch of young children were raised from infancy to adulthood, on an island, cut off from the world, they would Not become Christians. This was the rational basis for Christian missionary work.
To become Christians people would need to be told, or to read, about Jesus, his life and teaching - what it meant, what to do... People needed (one or more of) pastors, priests, a church organisation, access to scripture (aurally or by reading), some knowledge of the history of the church, knowledge of doctrine or theology... They may well require participation in specific (recorded, taught) rituals. That Kind Of Stuff...
But there is another way of regarding Christianity as a natural religion. I sometimes think of this as a 'cosmic' view of the work of Christ: that what Jesus did was to change the cosmos.
Jesus changed the nature of reality. He altered the structure of things - such that there were new possibilities; such that after Jesus people could be Christians - that is could follow Jesus to everlasting life in Heaven, and could become Sons and Daughters of God - entirely from their own natural, spontaneous, inner dispositions and knowledge...
Because of the cosmic changes made by Jesus; it became possible for someone to be a follower of Christ without any specific knowledge of Christ.
Is this true? I believe it is true; and I hope it is true!
Because if it is Not true, then Christianity is on its way-out; because the records and teaching of revelation, and the ways that people are interpreting it, are by now deeply tainted and corrupted.
Revealed Christianity nowadays points away-from Christ and towards the totalitarian System of this world. Revealed Christianity has it that Christianity must be changed and fitted into the mainstream, dominant, global ideology.
The question to ask is whether a loving God (our Father, the creator) would allow a situation to exist. Would such a God allow a situation in which his children - who wished to find it - were unable to discover and discern the truth; because the history was lies, the priests and pastors were political ideologues, the rituals were degraded, and everybody was trained to interpret the world through the lens of politics?
I do not accept such an understanding. On the contrary, I believe that God has made it so that whatever is required for Men to attain salvation and theosis is possible for any individual person, anywhere, unaided - and despite any amount of falsehood and distortion.
So, I now regard Christianity as one of the natural religions.
Note added: If you ask how? Consider that there need not be a single route to natural Christianity; indeed there very probably isn't. There may be as many routes to becoming a Christian (without 'external' revelation) as there are individuals; depending on their disposition and situation. There are innumerable possible ways that a particular person may come to know the truth - involving both innate qualities, and potential divine interventions.