Pagan is not a real religion. It describes a bunch of people who have religion- but in itself it is not one official and defined thing.
It's an umbrella name used by people like Christians to lump together many beliefs and rituals held by a wide variety of people. Christians for example would use 'Pagan' as a dirty word to slander non-christian religious practice and beliefs. When the 'pagans' existed back in their day- they didn't call themselves thus and didn't consider all of one and other to be in the same camp. They thought they were just doing what made sense, whether it was a blood sacrifice to ensure a crop yield or building a monolith to line up with astrological points. When they consulted a source for their religion, it didn't call itself 'Pagan' and it excluded many of the varying people and beliefs which are lumped into 'Pagan'.
To be a modern 'Pagan' is just someone who looks up in google and books and stuff what other 'Pagans' have done.. which comes from combinations of cultures and peoples and theories and presumptions, etc- all cobbled together.
You see how Mellowfarmer had to ask if it included one of his beliefs? See how he got tucked under an umbrella that could also contain people that don't celebrate the solstice? Notice how there was no holy book nor official source to cite if his belief is or is not 'Pagan'?