When reading anti-roman Catholic literature from the nineteenth century you sometimes come across the idea that the Roman Catholic Church taught the doctrine of "No faith need be kept with heretics". Was this ever true?
If these sources can be believed it was regarded as permittable to use lies, broken promises, fake offers of amnesty or safe conduct, treaties that were not intended to be kept or (most significantly) violations of the oath of loyalty to ones Monarch when battling against heretics. I have read of this being used as a justification for denying Catholics various rights in Britain on the grounds that their oaths of loyalty had no meaning.
I have heard the Council of Constance named as the Council that officially propogated the doctrine. I have also read that this was used as justification for the arrest of Jan Hus when he had been promised safe conduct. I have also come across sources claiming that the slaughter of the huguenots was justified with this idea.
Was this ever actually an idea in the Catholic Church itself? If so how widespread was it. I suspect that the anti-catholic nineteenth cantury sources might be exaggerating somewhat but I haven't managed to find any clear sources. What is the history of this idea whether as actual Catholic doctrine of Protestant propoganda?
It wasn't a universally believed or absolute doctrine that Catholics sowed no loyalty to Protestant rulers, or vice versa for that matter. However, it wasn't uncommon for some version of that to be espoused by individuals on individual cases, and for their generally to be a tension that the loyalty of someone outside the confessional status quo might be broken at any time. Given that religious dissidents were already often breaking the law via religious obligation and restriction, it didn't take much for authorities to distrust them nor for them to feel pushed to act back against oppressors.
If is worth noting that many religious minorities, Catholic or Protestant, still felt that monarchs outside their confessional genuinely held some legitimate authority. When it comes to Catholics, it worth noting how even examples like Edmund Campion, who defied the law in an audacious way, professed his loyalty to the Queen and was not involved in any assassination or armada plots. However, the papacy retained the papal power of deposition, which allow them to not only excommunicate rulers, but also such excommunicating bulls had the effect of denying the ruler's god-given authority. If you claim to have a divine right, a Catholic would be pretty skeptical of your claim when the Pope denounces it. Pius V's bull of Elizabeth I in 1570 specifically declared that oaths to her were non-binding and excommunicated those who obeyed here. Pope Gregory XIII, despite putting the bull into suspension, even said "whosever sends her out of this world with the pious intention of doing God's service not only does not sin but gains merit". The bull was later reissued by Pope Sixtux V around the time of the First Spanish Armada, although not many Catholics assisted that invasion because of their distrust of the foreign Spanish, as well as general hesitance around rebellion. Nonetheless, the retention of the papal power of deposition remained an issue during the reign of her successor, James VI and I who tried to convince the Pope to formally renounce it.
Similarly, there was also the political theory of tyrannicide, popular with religious dissidents of any confessional who were sufficiently bloodthirsty. This theory argued that a sufficiently terrible ruler, especially a heretic as it was mostly used in Reformation religious conflict, could be deposed. This gave a sharp theory behind the papal power of deposition, proving that there were some people who took it very seriously. The Gunpowder Plotters in 1605 invoked tyrannicide for example, as does Pius V's bull in reciting Catholic repression measures. As long as proponents of tyrannicide co-occurred with the papal power of deposition, Protestant states could never really feel safe. It didn't that many plotters in the home country when Hapsburg Spain was willing to support them, as it did with Balthazar Gerard and the Ridolfi Plot.
Furthermore, there was the principle of equivocation. A more peaceful, law-abiding rule that allowed Catholics to dissimulate, conceal things and tell half-truths, as long as they weren't technically lying to authorities. It became a big issue in English pop culture and academic discussions when Henry Garnet, Jesuit superior in England, was tried for concealing fore-knowledge of the Gunpowder Plot that he received via the seal of confessional. He practised and evoked equivocation in his actions and his trial. To the authorities obviously, this was nothing more than away for Garnet to pretend to be peaceful while complicitly allowing, even encouraging, the plotters. It's referenced in Macbeth—a play almost certainly written to flatter James VI and I, the intended prime target, and probably written soon after the plot—where there is a joke about an equivocator being in hell.
So there was an element of this, but not necessarily a singular and universal doctrine.