Eartheism is an alternative to spiritual/metaphysical religion. An appreciation and love of the Earth and all it's inhabitants: plants, trees, mountains, rocks, rivers, animals, bugs, and...okay, humans, I guess.
Eartheism is probably closest to the Native Americans' idea of co-existing with the Earth, to sustain life; that of themselves, as well as the Earth's, without overtaking it and destroying it. I suppose it shares some beliefs with Paganism and Wicca, but without the rituals and magic spells.
There are no restrictions to becoming an Earthiest; it transcends race, ethnicity, age, religion, politics, language. We all share the same house and walk on the same ground. You must only love nature and have a desire to see nature and wildlife preserved for future generations.
Unfortunately, some humans worship the Almighty Dollar and put profit before life. Some forget that we have the power to affect life here on our only home. We have the power to drive species to extinction, to pollute our atmosphere and oceans, to eliminate our finite supply of natural resources. The Earth is an organism, everything works(worked) like a well-oiled machine (pun intended). And we're throwing it all out of whack. We've broken the Circle of Life and made a necklace out of it. But, of course, we also have the ability to stop and stem the damage.
The overpopulation problem we face, which will only get worse, puts so much demand on the planet, that we must change our golbal consciousness into one of preservation, or soon we may face our own extinction.
The best way to be reminded of what we're losing is to visit the National Parks (and get out and walk, dammit!), hike your local rec. area, visit your local zoo/wildlife park, take your kids to the park, go camping, go for a bike ride, anything that gets you out of your house/office/car and lets you see/hear/smell/feel the natural world that many of us take for granted.

The founder/prophet of Eartheism is Edward Abbey (who'd hate being called that, I'm sure, and not only because it sounds like 'profit'), and this is dedicated to him. If you haven't read his books, do so immediately. Start with Desert Solitaire and Monkey Wrench Gang, and then read everything else.