Sixteen centuries ago, in AD 418, after a long and complicated series of debates over several years, Pope Zosimus finally issued a letter condemning as heretical the writings of Pelagius, the earliest known British writer.

Should we pay any attention to this anniversary, when so many others have occupied us this year? There were many heresies and theological disputes in the early centuries of the Christian Church – many of them incomprehensible even to practising believers today. Is Pelagius of more than antiquarian interest?

The prime reason for giving him attention is that Pelagius addressed a fundamental problem, not just a theological problem but a general and perennial one: free will. Because it was fundamental, the condemnation of his writings did not succeed. The Pelagian heresy was hard to kill, and the dispute continued to reverberate down the centuries. It resurfaced repeatedly – in Renaissance Italy, in the Reformation, in 17th-century Holland, England and France. Thinkers such as William of Occam and Jean-Jacques Rousseau were accused of Pelagianism.

No one knows for sure where or when Pelagius was born, but it was most likely around AD 360-370, somewhere in the British Isles. Some have suggested that his name could be a calque for the Welsh name Morgan, signifying “from the sea”, but that, too, is speculative. What is known is that by the end of the fourth century he was living in Rome and had acquired a following as a religious teacher.

Pelagius’s starting point was his response to what he regarded as an innovation by Saint Augustine. Sometime around AD 405, Pelagius heard someone quote the following from Augustine’s Confessions (then newly written): “Da quod iubes, et iube quod vis.” Augustine, addressing God in prayer, was saying: “Give what you command and command what you wish.”

Pelagius found this an unacceptably supine role for man in his relationship with God, and he disputed it. The argument broadened to include related questions of original sin, predestination and the significance of baptism, but seems initially to have been relatively amicable. Augustine recognised that Pelagius was a decent Christian and a serious thinker.

The central dispute was Augustine’s contention that a Christian could be saved only through God’s grace, unaffected either by good works or by the sins he or she had committed. Without grace, the sin of Adam in the Garden of Eden, eating from the tree of knowledge (original sin) condemned all mankind to hell. Pelagius retorted that through freedom of will, we could choose not to sin, if assisted by grace. For him, God’s creation was good, and he had made mankind good in his image. The idea of salvation by unearned grace alone, predestined before birth, was incompatible with the idea of a loving God.

Pelagius stood for free will; Augustine for original sin. As the controversy intensified, Pelagius was misrepresented and outmanoeuvred: his enemies alleged, inter alia, that he had said grace was not necessary for salvation at all. Augustine’s ally Saint Jerome abused Pelagius intemperately as a “huge, bloated Alpine dog, weighed down with Scottish oats”. In 418, after Emperor Honorius demanded action when people calling themselves Pelagians rioted in Rome, Pope Zosimus declared Pelagianism heretical. For those in power, a doctrine that persuaded the mass of ordinary Christians that they were unworthy, powerless supplicants, both temporally and spiritually, was useful. Augustine’s interpretation of original sin, closely associated with sexuality, also facilitated the misogyny of the medieval church.

But Pelagius has had the last laugh, in the liberal, humanist culture of western Europe today. Generally, we believe in free will, in the perfectibility of mankind, in the ability of people to make the right choices, do good, and to make things better (Pelagius was admired by the late Tony Benn). Many contemporary clerics in Christian churches in the West could fairly be called Pelagians. He might deserve that fourth plinth in Trafalgar Square.

Yet even if we share Pelagius’s distaste for Augustine’s views, and for their consequences, we should recognise that Pelagianism also has a downside. For Christians, an extended Pelagian system of belief might seem to leave little role for God at all, beyond the initial creation.

For others, who have observed the latter-day followers of Comtean positivism in education and elsewhere zealously impose ever more complex and impossible systems of measurement, process and compliance, an Augustinian position expresses some inescapable, if unwelcome, truths – as notably argued by John Gray.

An Augustinian awareness of our flawed nature, of the law of unintended consequences, of what Kant called “the crooked timber of humanity, from which no straight thing can be made”, is a much-needed corrective. Liberal humanism has its bigots too.