There is a phrase that has a lot of currency among Labour MPs who hate the direction their party has taken under Jeremy Corbyn. They say they will “stay and fight”. The vow is sometimes taken in private, sometimes declared on social media alongside a plea for others to do the same. It has become so familiar that its meaning is rarely interrogated.

But what is this fight? Who is it against? What does victory look like? The answers are not comfortable for the stayers. The enemy is the elected leader of their party and he enjoys the overwhelming support of its members. Corbyn’s position is so unassailable that his would-be assailants hardly dare to criticise him. They grumble about Labour’s position on Brexit and agitate for the endorsement of a second referendum. They express horror at the way antisemitism has eaten into the party like gangrene and demand surgical intervention. But in public, the dissenters avoid pointing blame squarely at the leader or even at individuals in his entourage.

The fact that they have taken action exposes the passivity of the remaining anti-Corbyn rump

In other words, the fight is against the way Labour is going, but not explicitly against the people who are taking it there. It is for a change of leadership – in theory – but only once the incumbent steps down, whenever that may be. Until this week, the best defence of that position was the absence of alternatives. Then seven Labour MPs quit to set up an “independent group” in parliament. The schismatic group might not look electorally viable, their plan might be ramshackle, their agenda obscure – but the fact they have taken action exposes the passivity of the remaining anti-Corbyn rump. It also flushes out tricky questions about what Labour is really for.

Those who find such questions uncomfortable hide behind the image of a “broad church”. This is another boilerplate phrase whose meaning wilts under scrutiny. When does a church become so broad that its congregants no longer profess the same faith? For Labour, that challenge goes beyond the present rows over Brexit and antisemitism. It drills into deep ideological faultlines.

Corbyn’s party has no leftmost boundary. There is no form of radical socialism that it deems taboo. It welcomes people who wave hammer and sickle flags, whether they are unaware of atrocities committed under that banner or simply relaxed about them. It is not controversial in the Labour leader’s office to see the fall of the Berlin Wall as a sad event. Corbyn’s inner circle includes former senior Communist party members and Stalinists.

A lot more than seven Labour MPs think Britain would be badly governed by such people and that the levers of state power – the army, police and security services – must never come under their hands.

The usual defence against charges that the party has been captured by extremists is to wave the 2017 election manifesto. It pledges nothing more sinister than a spot of light renationalisation, which is meant to prove that the whole project would look centrist by the standards of continental Europe. If it seems ultra-left it is only because Margaret Thatcher sent Britain hurtling off to the right. Many Labour moderates suspect the trajectory is to a darker place, and it is more than a hunch. Their view is based on the Corbyn’s past associations, familiarity with the tactics of the hard left at local party level and the invective of online trolls. But that is thin evidence in the court of members’ opinion. The leader’s testimony as a mild-mannered peacenik is more persuasive. So the question of what Corbynism really means has been parked. The model could be anywhere between Venezuelan socialism and Swedish social democracy. It can sound revolutionary for whipping up passions at a rally and reasonable for reassuring swing voters. It wants to abolish capitalism at the demo but only to reform it on the doorstep.

Those are not pillars supporting the same roof of a broad church. They are rival conceptions of what a government is for. Only in opposition can the cracks be painted over with gooey red gloss from the tin marked “Labour values”. This is the stuff that non-Corbynites reach for when they want to indicate their belonging to an institution that no longer belongs to them. Its raw material is moral certainty that progress and social justice are the exclusive property of the left, represented uniquely by Labour. It follows that any government sporting Labour branding is better than any alternative.

That view has been borne out often enough by appalling Conservative administrations, but it breeds ethical complacency. It implies that the merit in any idea – or person – can be dismissed simply by applying the label “Tory”. That is also a device for enforcing party discipline. Fear of ostracism keeps many unhappy moderates feigning loyalty to the church of Corbyn. (But excommunication is no threat to those who have renounced the faith.)

There is another flaw in the axiom that left is always good. It invites the conclusion that lefter must be even better and leftest the very best. It is a formula for romanticising dangerously extreme politics. Seven ex-Labour MPs this week refused to indulge that trend any longer. Their decision has been mourned as tragic by some former colleagues and denounced as wicked by others, which is always the response of believers to apostasy. But the split is just the expression of a banal truth: parties cannot be infinitely broad. People with incompatible views on politics, economics and history cannot all be right about what Labour represents. It is ridiculous to campaign for your party while dreading the prospect of its leader becoming prime minister. It is possible to stay with Corbyn or fight Corbynism, but not both.

• Rafael Behr is a Guardian columnist