Evangelical author and speaker Philip Yancey has slammed pastors who support Donald Trump, saying he is "staggered" that Christians see him as a hero.

Yancey described the Republican presidential candidate as a "bully who made his money by casinos" and "a person who stands against everything that Christianity believes". He pointed out Trump had "several wives and several affairs" and said he was baffled by evangelical support for the New York billionaire.

An excerpt from an interview with Evangelical Focus was released last week and Yancey warned the church in America against getting too involved in politics. He said lessons could be learnt from the European church's relationship with power.

"There are countries in Europe where the church is set back for decades and decades, because they have been stained by how they sold their soul for power," he said.

Yancey has written dozens of books including What's So Amazing About Grace, The Jesus I Never Knew and Christians and Politics: Uneasy Partners. But unlike a number of US pastors who lament the decline of Christianity in the West, Yancey said as the US becomes more secular, Christians become "more like the fertile soil in which the early church did best".

He said in the interview: "When you are in a place like the United States or Europe in its recent past, where the majority will claim to be Christians, but then they look like everyone else, then the people do not understand what the gospel is.

"But when Christians look radically different from the world around them, then the people can see the difference."

A number of prominent evangelical leaders have publicly endorsed Trump, including Jerry Falwell, president of Liberty University, Wayne Grudem, the Southern Baptist theologian, and James Dobson, founder of Focus on the Family.

Fewer evangelicals have backed Clinton, reflecting her difficulty with a voting bloc that could prove decisive. Author Max Lucado wrote a critical post of Trump but didn't say he would vote for Clinton. Deborah Files, executive advisor to the World Evangelical Alliance, is a lone voice to say she will vote Clinton.

"Hillary Clinton is the leader who people of faith are looking for and we are praying that Sister Hillary and not Mr. Trump will be elected in November," Files said in June.