Graham Greene, who wrote so much about Catholicism in his novels, was regularly asked whether he was still a believer. It was listening to the gospel accounts of Jesus’s life, he replied, and hearing fleeting references thrown into the narrative that had no obvious purpose for being there – like the “other” unnamed disciple who sprints past Peter on his way to the empty tomb – that tempted him to believe the gospels might just be fact.

At least that speedy but redundant disciple is there in plain sight. In God’s Ghostwriters, Candida Moss argues there are many other figures passing all but hidden through the pages of the New Testament, who actually created and shaped these foundational texts of Christianity. In St Paul’s Letter to the Romans, she points out, the closing chapter contains a discordant line that I must have heard many times before but never clocked: “I, Tertius, the writer of this letter, greet you in the Lord.” Isn’t the writer supposed to be Paul?

The name means “third” in Latin, explains Prof Moss, who is the Edward Cadbury chair of theology at Birmingham University, and he was probably the latest in a line of scribes who wrote down the great man’s thoughts to save him the bother but didn’t merit a name of their own. Why not? Because, she suggests, he was almost certainly an enslaved person, one of many thousands in the Roman empire who found employment as recorders and editors.

These ghostwriters were conscripted from among the educated in conquered societies, those who had been taken into slavery and sold at markets around the empire. And if they subsequently had children of their own, these too would be sent to “slave schools” to prepare them to follow in their parents’ footsteps as notaries, lectors, stenographers and secretaries, their accomplishments exploited by those who owned them for their own profit.

And if the Roman family that purchased them as a scribe had subsequently converted to Christianity, either openly or secretly as many did in the first and second centuries CE, they may well then have been drafted in to write down the words of the great Christian missionary preachers who criss-crossed the empire and came to its capital, including of course Paul.

Moss even finds a reference to one such scribe in some second-century CE graffiti found on the walls of what was once just such a slave school close to the heart of Roman power on the Palatine Hill. She imaginatively recreates the story of a boy named there, Alexamenos. He is depicted in the graffiti image standing at the foot of the crucifixion of his Christian lord, shown as was the Roman way at the time as having the head of a donkey to damn him as servile and stupid.

Alexamenos’s future, she suggests, most likely panned out as becoming secretary or scribe to well-to-do Roman families prominent among those who had converted to Christianity. He may even have written down some of the later Christian texts. By what Moss labels “slippage, corruption and alteration” within Christianity, the vital contribution of such individuals has been downgraded, sanitised and expunged, a deception that surely sits uneasily with the gospel message of justice.

Slavery, she alleges, thereby underpins the core texts of Christianity. It is a powerful claim to make at a time when the Church of England is seeking to create a £1bn fund to address the legacy of its involvement in the Atlantic slave trade. And it throws up other intriguing questions, too.

There is a long tradition in Christianity that presents the four gospel writers as close associates of the apostles, recruited to record first-hand memories of Jesus, which they did happily out of conviction, choice and love. What if, instead, they were enslaved individuals with no chance to refuse the task? Especially when in the case of Paul’s ghostwriters, they had to transcribe passages where he states his approval of slavery?

And to go one step further, the Mark of Mark’s gospel, the earliest of the four committed to papyrus around AD70, is usually said to be St Peter’s right-hand man. Could Mark have been an enslaved person? Perhaps when shaping the illiterate fisherman Peter’s words into an acceptable written form, this Mark might either consciously or unconsciously have infused them with some of his own experiences.

Big questions, almost too much to digest in a single reading, whatever the pleasure of Moss’s refreshingly readable (as in non-typical-academic) prose style. Yet definitely one to ponder the next time someone tries to tell us what they are saying is the “gospel truth”.

• God’s Ghostwriters: Enslaved Christians and the Making of the Bible by Candida Moss is published by William Collins (£25). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply