Donald Trump questioned the sincerity of Hillary Clinton's faith as he warned America's leaders were "selling Christianity down the tube" in a private meeting with evangelical leaders on Tuesday.

The presumptive Republican nominee asked for the support of several prominent faith leaders as he called himself a "tremendous believer" and assured them: "I'm so on your side". In clips of the behind-doors meeting posted on social media, Trump also boasted of picking up "massive majorities" of evangelical votes.

"The thing about Hillary in terms of religion is that she's been in the public eye for years and years, and yet there's nothing out there," he said in one clip. "There's like nothing out there. It's going to be an extension of Obama because with Obama you had your guard up. With Hillary you don't."

The New York billionaire went on to tell the leaders not to be politically correct and pray for all American leaders because many had damaged Christianity.

"People were saying pray for your leaders, and I agree with that ... but what you really have to do is pray for everybody to get out to vote for one specific person.

"And again, we can't be politically correct and say pray for all our leaders because all of your leaders are selling Christianity down the tubes, selling the evangelicals down the tubes, and it's a very, very bad thing that's happening."

The meeting is one of a number of efforts by the Trump campaign to secure the support of key Christian leaders who command widespread influence in the US.

Trump's newly announced evangelical advisory board is one such move and is expected to "provide advisory support to Trump on issues important to Evangelicals and other people of faith in America," according to a statement from his campaign.

The 25 members were "not asked to endorse Mr. Trump as a prerequisite for participating on the board."

"But Christianity; I owe so much to it in so many ways through life, through having incredible children. But I also owe it for, frankly, standing here," said Trump.

Outside Trump's event, other Christians protested his bid for evangelical votes. Rev Emily Scott, a pastor who joined the protest, said: "My opinion is that you cannot support Trump and also support Jesus," according to Business Insider UK. 

"I think that we know a lot about Donald Trump's religion, and it has a lot to do with hatred and greed, which is contrary to biblical teachings."