Approaching Earth Day on April 22, Americans might want to consider how environmentalism is becoming a new form of religion. They also might want to ask: Why is it OK to teach environmental religion in public schools while the teaching of Judaism, Christianity and other traditional religions is not constitutionally permitted?

Environmentalism has, indeed, become an article of religious faith. As Joel Garreau, a former Washington Post editor, wrote in 2010, "faith-based environmentalism increasingly sports saints, sins, prophets, predictions, heretics, sacraments and rituals."

Some argue that a religion must have a God, disqualifying environmentalism. Yet, as the American psychologist and philosopher William James observed in his 1902 classic, "The Varieties of Religious Experience," it is not necessary to "positively assume a God" in order to have a religion. James said "godless or quasi-godless creeds" also can qualify as religions, which - given its devout belief system and the fervor of its adherents - clearly would include today's environmentalism.

Paul Tillich, the greatest American theologian of the 20th century, defined religion as a comprehensive belief system that seeks to answer questions of "ultimate concern" to human existence. For Tillich, it was characteristic of our time that "the most important religious movements are developing outside of (official) religion."

The Supreme Court endorsed such an understanding of religion in the 1960s. In United States v. Seeger, involving the requirements for a conscientious-objector exemption from the military draft, the court ruled that the exemption should be applied equally to those who believe in a supreme being and those "with a sincere and meaningful belief which occupies in the life of its possessor a place parallel to that filled by the God" of religious believers.

Environmentalism borrows to a surprising degree from Jewish and Christian history.

It says in Deuteronomy that, for those who worship false idols, God "will send disease among you … fever, infections, plague and war … (and) will blight your crops." In 2010, Al Gore similarly foresaw environmental sinners headed toward calamity on a biblical scale, facing rising seas, "stronger and more destructive" hurricanes and droughts "getting longer and deeper."

In contemporary environmentalism, the largest religious debts are owed to Calvinism. John Calvin wrote that God has "revealed himself and daily discloses himself in the whole workmanship of the universe." For Calvin and environmentalism, the natural world is the artwork of God.

Man's role is to conserve God's work. Thus, the rituals of environmentalism celebrate reduced consumption - lowering the heat, driving fewer miles, using less water, living in smaller houses, having fewer children. Limiting human appetites, rather than satisfying ever-growing demands, is the environmental command.

The issue posed by environmentalism today for those who believe in the separation of church and state is: Does it make sense constitutionally to prohibit the teaching and embrace of Judaism and Protestantism in official public settings, while permitting the government establishment - as taught in the public schools - of this new secularized Protestantism: the religion of green, the religion of Earth Day?

Nelson is a professor of environmental policy at the University of Maryland.