The Germans have a wonderful word: Ohrwurm — literally, ear-worm — for a tune you cannot get out of your head. Recently, my Ohrwurm has been “Amazing Grace”: “I once was lost, but now am found, Was blind, but now I see.” My family is sick of me singing it at top volume.

“Amazing Grace” was written by John Newton, along with other hymns, such as “Glorious things of thee are spoken” and “How sweet the name of Jesus sounds”. As we shall often hear in this abolition-anniversary year, Newton was a slave-trader who had a conversion to Evangelical Christianity during a sea storm in 1748, and eventually set about arguing for the emancipation of slaves. “Amazing Grace” has been one of the anthems of Christian defiance ever since.

Yet what is gob-smacking about Newton is what a nasty piece of work he once was. The idea that the author of “Amazing Grace” raped his slaves has been a conceptual Ohrwurm for me ever since I found it out. Indeed, Newton’s conversion might have stopped him from swearing and drinking, but he continued to trade in slaves for six more years.

As Stephen Tomkins puts it in his new biography of Wilberforce, who was inspired by Newton, he “would read the Bible and pray for an hour or two, leading services for the crew, while his human cargo lay or sat hunched and chained under their feet”. It is astonishing to us, but the truth is that many Christians supported slavery because it was there in the plain meaning of scripture.

Slavery was given foundational justification in the book of Genesis, the curse of Ham condemning Ham’s descendants to perpetual captivity. It would have been seen as what contemporary Evangelicals call “a creation ordinance”.

The New Testament enthusiastically takes up this theme, for example in Paul’s first letter to Timothy: “Let as many slaves as are under the yoke count their own master worthy of all honour, that the name of God and his doctrine be not blasphemed. And they that have believing masters, let them not despise them, but rather do them service.”

What was deeply courageous about the Newtons and Wilberforces of the 18th century is that they fought their society’s prejudice, as well as the uncritical biblical theology that reflected it.

The idea that we might bask in the memory of these campaigners, without reflecting that there may be similar challenges for contemporary Christians, is to be radical 200 years too late. No: the spirit of “I was blind, but now I see” has a new challenge. And there are arguments within the life of the church today where we need to apply it.

Giles Fraser’s latest book, Christianity With Attitude, has just been published by Canterbury Press in the UK.