"There will be a storm of persecution against all people who follow God and His standards" if the US Supreme Court rules in favour of gay marriage, Franklin Graham has warned, and is now urging his Facebook followers to pray for each justice ahead of the decision.

"I don't want to sound like a broken record; but the most important thing we can do for any situation is to pray. God hears and answers prayer," Graham wrote to his 1.4 million fans on the social media site.

The president of the Billy Graham Evangelistic Association and Samaritan's Purse is a staunch opponent of same-sex marriage. He has previously described the Supreme Court's pending decision as "the most historic and far-reaching judgment of the 21st century," and warned that it "sets the stage for persecution of believers committed to living by the truth of God's Holy Word."

In a series of posts on Facebook, he has now asked for prayers for all nine justices; specifically that those who have opposed gay marriage in the past be given the courage to continue doing so, and those that have supported it change their minds.

Graham praised Justice Clarence Thomas, who was appointed to the Supreme Court by George Bush Senior in 1991, for having previously "voted to uphold the biblical definition of marriage".

"Let's pray especially for him today – that he will continue to stand as a voice of truth and wisdom in the face of vehement opposition from gay rights advocates," Graham said in a post yesterday. It had received more than 43,000 likes at the time of writing.

He also welcomed Justice Samuel Alito's suggestion that ruling in favour of gay marriage could have huge implications in the future, such as calls to permit polygamy. "Today join me in praying that Justice Alito will have wisdom and stand strong for what we know is God's unchanging truth-- Marriage is between 1 man and 1 woman," he wrote.

Justices Elene Kagan and Sonia Sotomayor did not receive such praise, however. Graham criticised their respective decisions to strike down the federal Defence of Marriage Act, which affirms that marriage is between a man and a woman. He urged his followers to pray that Kagan would "see the danger of deciding to go against God's law and standards", and that Sotomayor would "have the wisdom to know that as a society we cannot survive if we turn our back on God's standards and His definition of marriage."

The Supreme Court's ruling is due next month, and tensions are rising ahead of the decision. The majority of states – 37 out of a total of 50 – already permit gay marriage, and it is thought unlikely that American's highest court would reverse this course. Public opinion polls reveal large increases in support for same-sex marriages over the last ten years, but the final result will put an end to over two decades of litigation.

As expected, Christian groups are divided over the issue. A number of key leaders including Governor Mike Huckabee and Senator Rick Santorum have signed the Defend Marriage pledge, which states a desire to "defend marriage for what it is, a bond between one man and one woman, intended for life, and open to the gift of children." Jim Bob and Michelle Duggar, of 19 Kids and Counting, are also signatories.

However, there is a growing faction of Christians now in support of same-sex unions. A landmark vote in March saw the largest Presbyterian denomination in the US change the wording in its constitution to approve same-sex marriage. The marriage contract is now explicitly referred to by the Presbyterian Church (USA), which has some 1.8 million members, as "between two people, traditionally a man and a woman" rather than merely "between a man and a woman".

Other denominations that have signalled a move towards full acceptance of same-sex relationships include the Episcopal Church, which has approved blessings for gay marriages, and the Evangelical Lutheran Church in America which allows the ordination of gay clergy.