US President Donald Trump has tweeted his approval of a new book about him by two conservative evangelical commentators.

The Faith of Donald J Trump: A Spiritual Biography is by David Brody, a reporter for the Christian Broadcasting Network, and Scott Lamb, Baptist minister and columnist for the Washington Times.

They say in the book: 'As president of the United States, Donald Trump has been put in a position of authority by Almighty God, which makes the narrative about his faith even more vital to understand the man who promises to "Make America Great Again".'

Trump tweeted earlier today: 'The Faith of Donald Trump, a book just out by David Brody and Scott Lamb, is a very interesting read. Enjoy!'

The book includes interviews with Trump and his vice-president Mike Pence about Trump's religious beliefs and relationship with evangelicals. Pence says: 'President Trump is a believer. I say that with great conviction. I think his faith in God and his faith in the American people are the foundation of his life and service.'

The book claims: 'President Trump won over evangelicals not by pandering to them, but by supporting them and all their most important issues without pretending to be something he's not.'

The foreword by conservative writer and activist Eric Metaxas acknowledges the title raises suspicions of parody, but says: 'The simple fact... is that Donald Trump has demonstrated two things that typically don't go together at all. One the one hand he has been tremendously popular with evangelical Christians and outspoken in his defense of faith in public life; on the other hand, he has evinced a startling lack of familiarity with the Bible and has even admitted being unable to recall ever asking for God's forgiveness.'

He also refers to Trump's much-publicised moral failings. However, he says, widespread Christian acceptance of him as president is because 'the God of the Bible does not ask us to be morally perfect so that he will accept us'.