What the text says. Pilate tells the accusers to "Take him yourselves, and judge him according to your law", and they answer, "It is not lawful for us to put any man to death" (John 18:31, ASV).
The legal fact behind it. Judea after AD 6 was governed by a Roman prefect, the title confirmed on the Pilate stone from Caesarea, which reads praefectus Iudaeae, and capital jurisdiction in a Roman province rested with the governor. The clearest independent confirmation is an incident Josephus reports for about AD 62: the high priest Ananus, in the gap between the death of the procurator Festus and the arrival of his successor Albinus, convened the Sanhedrin and had James, the brother of Jesus, stoned. Leading citizens protested to King Agrippa and went out to meet Albinus on the road to tell him "that it was not lawful for Ananus to assemble a sanhedrim without his consent". Albinus wrote to Ananus in anger, and Agrippa removed him from the high priesthood after three months (Antiquities 20.9.1). A Jewish court that executed a man without Roman consent lost its high priest over it. That is why the case went to Pilate: crucifixion was a Roman penalty, carried out by Roman soldiers under Roman authority, and the death sentence was the governor's to give.
What is disputed. How absolute the restriction was is debated. Against a total ban stand the stoning of Stephen (Acts 7:57-60), which reads like an execution without a governor, and the temple warning inscription, a first-century Greek notice from the Jerusalem temple, of which two copies survive, forbidding any foreigner to pass the barrier on pain of death, which appears to reserve one capital penalty to the temple authorities. A rabbinic tradition states that capital cases were taken from Israel forty years before the Temple's destruction (Jerusalem Talmud, Sanhedrin 1:1, 7:2), which is late evidence and is read both as corroboration and as retrojection. Scholars have argued this both ways for a century and this file picks no winner. The point that survives the dispute is the one John's account turns on: for a routine capital charge in Judea, Rome held the sword.
Sources: John 18:31; Acts 7:57-60; Josephus, Antiquities 20.9.1; ISBE 1915, "Pilate; Pontius" and "Sanhedrin"; Jerusalem Talmud Sanhedrin 1:1 and 7:2; the Pilate stone and the temple warning inscription.
---