This is not a belief I share at all--I follow the older pre-christian paths, and as Vegas Elias has said, there are numerous christian belief systems that were far older customs, and were "baptized" as it were, by the Roman church, in order to make it easier for them to force conversion onto the pagans.
(Pagan, by the way, is a very old word, and means only "country person," or "country dweller," possibly in much the same way we'd currently use the term of someone being a "country bumpkin.")
It does NOT mean devil worship or any other such negative connotation that it has been assigned.
With that clarified, however, if that is what you believe, it is as simple a concept as a person who is able to be at once a daughter, a wife, a mother.( and.even a sister; now you have 4) "different" people who are all the same person. The same goes for men--just shift it to 'son, husband, father, brother.'