The following is an idea for a fictional religion that I had.
A Celtic druid in pre-Roman times by the name of Pendrogis thinks long and hard about it, and formulates his own theory about the nature of reality. He and his followers founded a monastery, and that is the start of the religion called Pendrogism, which eventually becomes dominant in his country.
Core Tenets:
All human souls were once one. However, this united being was formless and had no experiences. Therefore, it divided itself into countless pieces which indwelled the members of the species Homo Sapiens, therefore granting human beings a soul, an immortal aspect. All people's consciousnesses are souls, having stemmed from the unified being, and the body is simply an instrument with which we're able to perceive and interact with this reality.
The fact that all people are different and separate means that all consciousnesses are different also different and separate. Our own being the only perspective we experience, humans are inherently selfish. Selfishness is the root of all human behavior. There is no good or evil, only selfish acts which have positive effects on other people and acts which have negative effects on other people.
At some point, every person is going to die. When we die, our souls will attempt to reincorporate into the unified being. As part of it, you truthfully perceive yourself to be the unified being, and you think its thoughts, and so does everything else at the same time. The purpose of this being iz to engage in a dialogue with the God who is unknowable to the inhabitants of the material world, so that the two can understand each other and merge into one. God is infinite, and over time an infinite number of people, each contributing their unique life experiences into the mix, will join the unified being, so the process of dialogue between God and the unified being will go on eternally.
However, in order to join it we must first overcome our selfishness, because each individual selfish perspective is incompatible with the unified being. For some individuals, this takes centuries or even millennia to figure out how to do. Other individuals are so destructively selfish, and they typically came to be this way through poor life choices, that they are totally incompatible with the unified being and thus are cast into "the outside", where they are alone for all eternity. These are typically people who live selfish lives that cause harm to other people, the most extreme examples being serial killers and child molesters.
However, a person can immediately be incorporated into the great being upon death if they master it in this life. It is not enough to simply know the right stuff, or to just BS it and force yourself to act selflessly your whole life. You must look past a person's material body and into their soul, and experience the fact that their consciousness, while different from yours, is also the exact same. You learn to experience what they experience, and you'll naturally come to see what is good to and for them to be equally important as what is good to and for you, because you experience both. In short, the term "selfish" is redefined by redefining the "self". The easiest and most common manifestation of this relationship is between husband and wife; if you achieve such a relationship with just one person in this life, then when you die joining the unified being will be a quick and simple matter.
