On this day in 1553, the condemned heretic Michael Servetus was burned at the stake in Geneva, largely thanks to the efforts of theologian and Protestant reformer John Calvin.

Though he remains popular and influential today, Calvin's involvement in this execution remains a dark mark on his past, and an unforgettable reminder of how theology can turn sinister.

Calvin and his landmark Institutes of the Christian Religion have left an undeniable impression on the modern world. Reformed theology is indebted to his legacy and has inspired popular 'neo-Calvinist' evangelical pastors like John Piper, Tim Keller and groups like The Gospel Coalition. Calvin's thinking emphasised themes such as human depravity and a total reliance on God's sovereign grace, not earned or chosen by human beings but predestined for (some of) them by divine election.

Calvin was a towering intellectual and devoted student of Scripture – but at least one moment has blighted his legacy.

It's chronicled well by historian Nick Page in his recently published A Nearly Infallible History of the Reformation (Hodder & Stoughton, £18.99). The Spanish physician Michael Servetus was not a popular man – in fact he was a widely despised heretic – a rare point of agreement for Catholics and Protestants in the 16^th century. He was a Unitarian, which means he didn't subscribe to the doctrine of the Holy Trinity, a historic tenet of Christian orthodoxy. He saw no such doctrine in Scripture and felt that rejecting it would provide significant unity between Christians, Jews and Muslims.

Calvin and Servetus corresponded for a time before the former gave up in frustration. He warned that if Servetus came to visit him in Geneva, 'I would not let him leave alive'.

He wasn't exaggerating. Calvin informed the Catholic Inquisition of Servetus location – they decreed that he be burned, though the lucky heretic escaped his capture. He was caught out when he came to Geneva however, where he was spotted and the local council had him arrested and sentenced to death. To give some credit to Calvin, he encouraged Severtus to repent, with no success. He also unsuccessfully lobbied for some (relative) leniency for the prisoner, suggesting beheading instead of being burned alive.

But various correspondence shows that even though Calvin didn't sentence Severtus, he still believed it was right for him to die for his heresy. As for those who criticised his enthusiasm for Servetus' death, Calvin revelled in their opposition – and said they were just as guilty as the heretics.

One contemporary of Calvin's, Sebastian Castellio, said that his hands were 'dripping with the blood of Servetus'. Even though Servetus would have probably faced death without Calvin's help, it's not an unfair assessment.

The glee of Calvin (and those around him, like Scottish reformer John Knox) at the death of a heretic remains deeply troubling, even with historical accommodation granted. It displays a remarkable lack of mercy and is a far cry from humble 'good disagreement'. Even though the threat of death for heresy is rarer today, it's not hard to see a similarly confident, antagonistic conservatism in the Church that's still alive and well. In a choice between being loving and supposedly 'correct', many Christians favour the latter.

Even Calvin devotees accept this was not his finest hour. The death of Servetus stands as a dire warning to warring theologians.

In the 500^th anniversary of the Protestant Reformation, its important to celebrate all the good that came of the era. It's also crucial to mark that like anything in human history – but tragically tinged here with conservative Christian 'enthusiasm' – it has a dark side too.