Christian billionaire couple join Mark Zuckerberg and the Gates in giving away their riches

Hobby Lobby CEO David Green and his wife are among the latest billionaires to pledge to give a majority of their wealth to philanthropic causes.

The Christian couple joins Warren Buffet, Bill and Melinda Gates, New York City Mayor Michael Bloomberg, Facebook founder Mark Zuckerberg and at least 50 others in making the bold pledge.

Hobby Lobby, an arts and crafts store, was established in 1972 and has since grown into 462 stores in 39 states in the US. Its vision statements reads: "Hobby Lobby partners with organisations working to share the Good News of Jesus Christ to all the world."

"God has blessed me with a wonderful family, a successful business and outstanding employees. I do not take these blessings lightly," Green says in his pledge letter. "I am honoured to join this remarkable idea called The Giving Pledge."

The Giving Pledge is the brainchild of Buffet and the Gates. The idea of the pledge came out of discussions among philanthropists, all of whom shared how giving had made their lives richer.

The project was launched in June and invites the wealthiest American families and individuals to help address society's most pressing problems by giving away at least half of their wealth.

Buffet, the third richest person in the world, has pledged to give more than 99 percent of his wealth to philanthropy during his lifetime or at death.

The Giving Pledge, the billionaires stress, "is a moral commitment to give, not a legal contract".

"It does not involve pooling money or supporting a particular set of causes or organisations."

For David and Barbara Green, giving back has always been part of their business model.

"Coming from a family of preachers, the idea of giving back has been part of my life as long as I can remember," David Green says. "When Hobby Lobby was created in the early 1970s, I was committed to use profits to help ministry work."

Green has contributed to missions throughout the world, orphanages and ministries in the US, among many other things.

"Hobby Lobby," he stresses, "has always been a tool for the Lord's work."

"For me and my family, charity equals ministry, which equals the Gospel of Jesus Christ."