Third, that the custom of releasing a prisoner finds no attestation beyond the Gospels is said to be an argument from silence: “absence of evidence is not evidence of absence”.
A response is that one should expect to find such evidence of these supposed “customs”. Here custom indicates a repeated practice; consider Mark’s references suggesting Pilate’s releasing of a prisoner being a regular “custom at the festival to release a prisoner” and that “The crowd came up and asked Pilate to do for them what he usually did” (emphasis added); according to New Testament scholar Helen Blond, “Mark’s account implies that the amnesty was a custom which either Pilate had introduced himself or had inherited from his predecessors and seen no reason to discontinue” (10).
Yet such a custom is not attested in various independent sources from which historians learn about the Empire and its punishment of criminals. This absence of evidence charge is weakened and the story becomes suspicious, and especially so when it is combined with other factors in the Gospels, such as the apologetic motives, as well as the unlikeness of a governor like Pilate releasing an insurrectionist against the Empire.