An environmental group is asking Pope Francis to abstain from all animal products during Lent, promising a $1 million donation to a charity of his choice if he does so.

"Today, Pope Francis, I am asking you to join me in abstaining from all animal products throughout Lent, and to endorse the Million Dollar Vegan campaign," Genesis Butler wrote in a Feb. 6 open letter to the Roman Pontiff.

"Should you join me, the Blue Horizon International Foundation will donate $1 million to a charity or charities of your choice as a gesture of their utmost gratitude for your commitment."

While a vegan fast is not now prescribed by the Church, the practice would hearken back to practices of the early Church, and of the Christian east.

Butler, 12, is an animal rights and environmental campaigner. Her letter is backed by Million Dollar Vegan, a non-profit group which highlights the effects of animal farming on climate.

She recalled that in Laudato si', his 2015 encyclical on care for our common home, Francis "stated that every effort to protect and improve our world will involve changes in lifestyle, production, and consumption."

She also expressed her appreciation for his "speaking out on climate change, habitat loss, and pollution, and for reminding the world that Earth is a home we all share."

Butler said that "the current eating habits of predominantly richer nations are causing global destruction and devastation," as animal farming is resource-intensive relative to calories yielded.

The activist said that "moving towards a plant-based diet will have substantial environmental benefits." She said it would protect the environment, "help feed the world's most vulnerable," and "benefit human health."

An accompanying petition asking Pope Francis to try vegan for Lent and to encourage others to do the same has garnered more than 33,000 signatures.

This year, the Lenten season begins March 6; Easter Sunday will be April 21.

Under the 1983 Code of Canon Law, Catholics aged 18-59 are to fast on Ash Wednesday and Good Friday. And Catholics 14 and older are  to abstain from meat on all Fridays; this rule allows the use of eggs, milk products, and condiments made of animal fat.

But in times past, a vegan Lent would not have been so different from Catholic practice.

According to The Oxford Dictionary of the Christian Church, "during the early centuries the observance of the [Lenten] fast was very strict."

The reference work says that in the early Church the Lenten fast allowed one meal per day, taken towards evening, and that "flesh-meat and fish, and in most places also eggs and lacticinia, were absolutely forbidden," but that the practice "began to be considerably relaxed" in the west from the 9th century.

The 12th century Decretum Gratiani, a compendium of ecclesiastical law, includes the text of a letter which was believed by Gratian to be from St. Gregory the Great to St. Augustine of Canterbury. This letter says that during Lent "we abstain from flesh meat and from all things that come from flesh, as milk, cheese, eggs." A critical edition of the Decretum calls the source of the quote Pseudo-Gregory, and according to Dr. Mark DelCogliano, an associate professor of theology at the University of St. Thomas, the quoted text "first appears in Gratian."

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Writing in the late 13th century, St. Thomas Aquinas said that it was "common custom" that those fasting abstained from meat, eggs, and dairy products, but were allowed fish.

It is said that the practice of calling "Fat Tuesday" the day preceding Ash Wednesday derived from a period when the use of animal products was barred during Lent. "Fat" Tuesday was thus the last day to use up the meat, cheese, and animal fat stored in the home.

And while not precisely vegan, the traditional Byzantine fast barred the use of meat, dairy, eggs, and fish. Animal-based foods that were permitted included honey and invertebrates.

In the Byzantine rite, Lent is preceded by a pre-Lenten period known as Fore-Lent. The last two Sundays of this preparatory period are known as Meatfare Sunday and Cheesefare Sunday.

Under traditional fasting rules, Meatfare Sunday was the last day before Easter to consume meat, and Cheesefare was the last day to use dairy products. The Lenten fast then began on the Monday after Cheesefare.