Yesterday’s spy can be tomorrow’s incorrigible gossip. Take, for instance, Ján Sarkocy, the former Czechoslovak intelligence officer, who has become positively garrulous in the past week about his alleged contact between 1986 and 1989 with Labour MPs. Having already claimed that he paid Jeremy Corbyn for information to pass on to Prague and Moscow – a charge the Labour leader vehemently denies – Sarkocy has now told the Sunday Telegraph that he and his KGB counterparts ran a cell of at least 15 senior Labour figures during the 80s. Thus far, he has named John McDonnell and Ken Livingstone, both of whom emphatically reject the allegations of espionage.

Corbyn does not deny meeting a Czech diplomat, apparently Sarkocy using the name of “Lt Jan Dymič”. The Islington North MP was listed as source COB by the Czech secret service, Státní Bezpečnost, and is reported to have played host to his alleged handler at least three times at the Commons and his constituency office.

The cloisters of theology are not a sensible place to debate the merits of a prospective prime minister

There are two legitimate responses to these reports. The first, to which I incline, is that Corbyn and his friends on the Labour left were, at the very least, playing with fire at a time of great geopolitical tension. Such contacts, however innocent they may have seemed, were freighted with significance and peril, subtly encouraging the myth of moral equivalence between the two sides in the cold war.

For all its errors and terrible interventions, the west was always preferable to the totalitarian alternative. Ernest Bevin, the great Labour foreign secretary, understood this with admirable clarity. As he warned the cabinet in 1947: “Our belief in the human rights and in the liberties of western democracy is part of our way of life, which must be the basis of all our publicity. This belief represents in foreign countries more than anything else what we stand for in the world.”

Bevin’s point was that his own party had often failed to stand by this principle when confronted with the horrors of Soviet communism – a history of evasion and wilful blindness meticulously and incontestably chronicled in Giles Udy’s magisterial Labour and the Gulag. Four decades after Bevin’s remonstrance, Corbyn was cosying up to a dreadful regime that was still routinely imprisoning Václav Havel and his fellow dissidents. I can only speak for myself, but that rubs me wrong.

An alternative response is this: the threat of nuclear holocaust in the second half of the 20th century imposed a special responsibility on all western politicians to explore every possible opportunity to build bridges with representatives of Warsaw Pact nations. According to this analysis, Corbyn’s meetings with Sarkocy were entirely consistent with his lifelong commitment to dialogue and diplomacy. If the substance of his conversations was being reported to intelligence services in Prague and Moscow, so much the better. It was – so the thesis runs – vital that those at the apex of the communist apparat understood that many in the west sought peaceful resolution of the conflict, rather than the terrible gamble of mutually assured destruction.

So there is a debate to be had. Much more alarming, however, than either of these arguments is the mildly amused indifference which the Sarkocy story has generally inspired. In John le Carré’s most recent novel, A Legacy of Spies, George Smiley laments “a cause the world barely remembers” – and, as so often, he is right. The worst possible reaction to the reports of Corbyn’s Czech connection is also the most common: who cares?

Part of the problem is that any criticism of the Labour leader – of whatever character – is axiomatically dismissed by his followers as “a smear”. It is not just that they defend him, as any loyal supporters would. It has become central to their worldview that JC is, quite simply, beyond reproach. Here we leave the realm of political discourse and stray into the cloisters of theology – not a sensible place, I would say, to debate the merits of a prospective prime minister.

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But there is a greater and more pernicious force at work here, too – and one that is not confined to the Labour party. It is the notion that we live in an age of daily reboot, in which the past is of only antiquarian interest and certainly of little relevance to the present. The cold war? Didn’t that all end in 1989? Why drag that up?

To which the answer is – because the jagged legacy of that exhausting struggle does much to explain the predicament in which we find ourselves today. The pathologies of unrestricted globalisation; the terrible absence of a Marshall plan for the former Soviet bloc; the rise of religious fundamentalism; and the surge of nationalist populism in a world in which glacial confrontation has been replaced by unimaginably hectic upheaval: none of this makes sense without an understanding of what preceded it.

It is the gravest of mistakes to confuse change with a clean slate, and to allow modernity to become amnesia. By all means acquit Corbyn of the charge that his meetings with a Czech diplomat in the 80s make him unfit for the highest office in 2018. But don’t pretend that they are an irrelevance. Who controls the past controls the future, as all totalitarians know. But it is also true that those who insist upon understanding the past have the best chance of safeguarding both their own liberty and the prospects of progress. Be warned: the idea of “year zero” only has value to those with something to hide.

• Matthew d’Ancona is a Guardian columnist