The ancient world knew two classes of human, slave and free, who were so distinct as to be almost different species. Centuries later, the immediate post-Darwinian world often claimed that even if people were indeed from the same species, the different evolutionary routes they had taken has confirmed rather than eradicated their inequality. Even Darwin himself, driven by a hatred of racism and slavery and powerful moral and humanitarian concerns came perilously close to legitimising this view. "At some future period, not very distant as measured by centuries," he wrote in The Descent of Man, "the civilised races of man will almost certainly exterminate and replace throughout the world the savage races."

We should not, therefore, underestimate how counterintuitive and countercultural human equality is. Nor should we underestimate the extent to which our contemporary commitment to equality is drawn from biblical Christianity.

Genesis 1.26-27 – "… So God created man in his own image, in the image of God created he him; male and female created he them," – is perhaps the single most influential biblical text in the British political history.

It is not, on the surface, a political text at all. Yet, the manner it has been used has been repeatedly and controversially political. Here lay the basis of John Ball's aggressively egalitarian preaching – "When Adam delved [dug] and Eve span / Who was then the gentleman?" – that led, along with his role in the Peasants' Revolt, to a gruesome death. Here lay the basis for John Locke's more sophisticated and more influential First Treatise on Government, which took on and methodically demolished Robert Filmer's argument that political inequality was built into creation. Here lay the basis for the abolitionist claim that the slave trade was a monstrous abuse of God's own creatures. And here lay the basis – well, a basis – for Thomas Paine's cry of political emancipation: "If [Genesis] be not divine authority, it is at least historical authority, and shows that the equality of man, so far from being a modern doctrine, is the oldest upon record."

In reality, the imago dei was only one of the two main pillars for the Christian insistence on equality. The second was its New Testament counterpart: just as all are made in the image of God, all are saved – or, rather, are offered the gift of salvation – by the same God.

Once again, all meant all, in spite of what the educated and sophisticated might have wanted. Christ was disparaged for associating with low-lifes. He saw himself in them. St Paul intimated that the poor and ill-educated were especially open to the gospel, a fact to which Locke referred in his Reasonableness of Christianity: "[The sophisticated are] shut out from the simplicity of the gospel; to make way for those poor, ignorant, illiterate, who heard and believed promises of a Deliverer." Just as God made all in his image, so Christ offered all his image, to replace the broken original.

Perhaps more than any other political commitment, equality is the beginning rather than the end of a conversation. Centuries of Christian history show how the biblical conviction of irreducible human equality has existed, seemingly happily, with all manner of economic, political and legal inequality through the ages (although, perhaps not quite as much in Britain as polemicists intimate).

However, we would be advised to think carefully before pointing the finger too accusingly over this issue. Interviewing a philosophically minded peer last year, I was confidently told that in some cases abortion was not, in fact, a reluctant evil but an actual good because some babies were born to "girls … [with] no future anyway," and would simply grow up to be "educationally worse off … the underclass [will be] financially worse off, they're the people who don't get jobs, they're the people who suffer."

The interviewee subsequently asked me to cut "some of the rather rambling remarks about abortion." A line had apparently been crossed, beyond which no public figure would wish to be seen in public. But the whole exchange was suggestive of how alarmingly readily we slip away from the conviction that all, no matter where they find themselves on the social scale, are fundamentally, ineradicably equal.

This article first appeared in the Guardian.