Evangelical leader Franklin Graham has described the US Supreme Court's pending decision on whether to legalise same-sex marriage across America as "the most historic and far-reaching judgment of the 21st century".

He warned that a decision in favour will pave the way for the persecution of Christians and asked for prayers for the Supreme Court justices responsible.

Writing on his Facebook page, Graham, president of Samaritan's Purse and of the Billy Graham Evangelistic Association, said: "This is a pivotal moment in the history of America. As nine US Supreme Court justices consider arguments brought before the court on April 28 about whether to make same-sex marriage a constitutional right, we need to pray fervently for the wisdom and counsel of God to work its way into their hearts and minds of those making this momentous decision.

"The impending decision will rank as the most historic and far-reaching judgment of the 21st century. If our nation's highest court decrees same sex marriage as the law of the land, the consequences will be great. It sets the stage for persecution of believers committed to living by the truth of God's Holy Word. Join me in praying for each of the justices: Chief Justice John G Roberts and Justices Elena Kagan, Samuel A Alito, Jr, Antonin Scalia, Anthony Kennedy, Clarence Thomas, Ruth Bader Ginsburg, Stephen Breyer, and Sonia Sotomayor."

In another post, he wrote: "If same-sex marriage becomes the law of the land it could have a drastic impact on Christian universities, organisations, and churches. Those who operate by Christian principles could be severely impacted and even shut down. God hears our prayers, and we need to pray that He will work mightily in the hearts and minds of our Supreme Court justices right now as they consider this monumental issue."

He also commended Barronelle Stutzman, owner of Arlene's Flowers in Richland, Washington, who has been embroiled in litigation after declining to provide flowers to a gay couple because it would have gone against her religious beliefs.

Graham wrote: "I commend Barronelle Stutzman for living by the biblical principles of her Christian faith, no matter the cost. She says in the Washington Post, 'We've always heard that same-sex marriage would never affect anyone aside from the same-sex couples who wanted to be married. But a judge recently told me that my freedom to live and work according to my beliefs about marriage expired the day same-sex marriage became the law in my state.' Pray for our Supreme Court justices as they consider the issue of same sex marriage for our nation – today pray especially for Justice Clarence Thomas who has served on the court for over 23 years. I assure you this affects each and every one of us. There will be a storm of persecution against all people who follow God and His standards if this passes."

Same-sex marriage has been legalised in 36 states and the District of Colombia. The Supreme Court is deciding whether all 50 states should make it legal under the Constitution. The ruling is expected later this year.