Let's start from the alleged quote itself, as it is not a direct quotation but rather a short, rhyming paraphrase of an actual passage from the 11th letter in Questions sur les miracles that goes 'Certainement qui est en droit de vous rendre absurde est en droit de vous rendre injuste' ['Surely the right to make you absurd deeds is also the right to make you injust deeds']. In this particular letter, Voltaire discusses the plagues of Egypt as being a work of fiction because they weren't corroborated by any historical sources of Egyptian origin (Voltaire points to Manetho, an Egyptian chronicler living in 3rd century BC as well as to Cheiremon of Alexandria and Flavius Josephus), pointing out that the even though the duel between Moses and Pharaoh's wizards could have been a private affair, but the repercussions, such as plague and especially the death of all the firstborn across the Egypt could not have been omitted by historians (he also refutes the allegations that Egyptians were ashamed that they eventually lost, pointing that there is no precedence to such event, and neither Greeks have hidden their defeats in the Peloponesian War, nor the Romans tried to expurgate information about their defeats at Cannae or Lake Trasimene).

In the following paragraphs, Voltaire then states that although the miracles of Moses are not corroborated by historical evidence, it would be dishonest to state that they never happened, because we do not know this either and Christians, as well as Jews can only believe in them. But then he adds that if people start to believe in things they consider absurd and thus, as he puts it 'believe what they do not believe', they might forfeit their reasonable conduct. Then he makes the quoted allegation that anyone who can make people believe things that are absurd or injust, can easily make one commit actions that are as absurd or injust as said beliefs and people will do it gladly, to keep their action aligned with their convictions. This line of reasoning is quite interesting from the academic perspective, because it mirrors the idea of cognitive dissonance introduced by Leon Festinger in 1957 and more precisely, the mechanism of reducing said dissonance through taking an action only to preserve the congruity with the current attitudes, described in length by Elliot Aronson and David Mette in late 1960s.

In this letter, Voltaire does not mention any particular absurdities or injustices save for general allusion to 'civil wars' and 'tribunals of Inquisition', he simply notes that believing in thing that are absurd might easily cause people to commit acts that are equally absurd, when judged from outside, often without realizing what they are doing. This is also not an attack on the organized religion as such, but rather a warning that potentially benevolent religion can lead to immoral acts by misuse or misinterpretation of their own tenets. This is evident as he ends the letter wishing that the religion of his imaginary recipient (e.g. the reader) was good and congruous, being 'moral in theory and beneficial in practice' and thus grounded in rationalism rather than dogmatism.