To explain Christianity you need to take a step back in time, and consider the roles of the first superstitions and mythologies.
Early man didn't know much, and the thing about living in a world you don't understand is that for many beings it is scary. You can see this not only in humans, but in other species as well. For early man, legends of great ancestors, gods, or even simple stories of how an animal got its spots, or how the giant saber toothed wolf was defeated by a hunter would have been quite reassuring.
Add in the natural state of superstition, the idea that an action - however inane - can influence the outcome of an event, and you have the beginning of religion. To understand how pervasive superstition is, you can even see it in birds in the video 'The Superstitious Pigeon'.
Anyway, storytellers in early civilization were held in high regard, for they not only passed on legend, they passed on the knowledge of the generations before them. And this put them in a position of power. In some cultures there is little sign of abuse of this power (North American medicine men are highly regarded, but rarely in a position to abuse members of their tribe) while in others there is significant manipulation of the populace via superstition and religion.
The thing about religions is that they invariably create a false wall of superiority around those who practice them. "Our god" is better. "Our god" is true. "We" shall be rewarded for our obedience. "The unbeliever" shall be "cast out / punished / destroyed / etc.".
Whatever the first religion was, and we certainly will never know because it existed before written language, it evolved and splintered into many factions, modified by its adherents over time to fit the needs of the ruling class - often kings, clergy, or a mixture of the two.
Christianity is just one of those splinter religions, no more or less valid than the religions of Islam, Norse, Aztec, Egyptian, etc. etc.
And, in fact, you would be hard pressed to find two Christians who believe the same thing about their religion if questioned separately. Some believe the bible is merely a book of parables intended to educate, while others believe that it is the inerrant and literal word of "God". The latter seem to miss the part where the inerrant book is a heavily edited and oft-translated document bearing only some resemblance to the original Greek manuscript. But that's a whole other can o' worms.
On a planet where there are over 5000 identifiable gods recorded in history, the god of Christianity is just another face in the crowd, but one that has persisted thanks to ardent imperialism. We can thank the Romans and their Empire for this, not only for spreading it through conquest, but providing an Imperial framework which the church was based upon. Conversion is easy if you suppress the beliefs of the older generations, and plant a belief system in the young. Entire populations in a geographic area can be converted to a new belief system in two generations, and prior culture can be erased as quickly. You can see examples of this in South America, where methods of agriculture that have been handed down for thousands of years are dying out or lost altogether simply because new tools for creating monoculture farming, or imported trade goods eliminated the need for this information, and the elders sometimes died before the young realized they were losing the traditions of their people.
So to sum it up, the explanation for Christianity is that it is simply a successfully propagated belief system adhered to by a significant, but not dominant, percentage of the population.
yet nobody has seen the insights of God? 
. It keeps the earth warm and keeps everything growing. Without it.. we would be nothing ...
