5 But when the evangelist John adds, “That the saying of Jesus might be fulfilled, which He spoke, signifying what death He should die:” if we would understand such words as referring to the death of the cross, as if the Jews had said, “It is not lawful for us to put any man to death,” for this reason that it was one thing to be put to death, and another to be crucified: I do not see how such can be understood as a consequence, seeing that this was their answer to the words that Pilate had just addressed to them, “Take ye him, and judge him according to your law.”
If it were so, could they not then have taken Him, and crucified Him themselves, had they desired by any such form of punishment to avoid the putting of Him to death? But who is there that may not see the absurdity of allowing those to crucify any one, who were not allowed to put any one to death? Nay more, did not the Lord Himself call that same death of His, that is, the death of the cross, a putting to death, as we read in Mark, where he says, “Behold, we go up to Jerusalem; and the Son of man shall be delivered unto the chief priests, and unto the scribes; and they shall condemn Him to death, and shall deliver Him to the Gentiles: and they shall mock Him, and shall spit upon Him, and shall scourge Him, and shall put Him to death, and the third day He shall rise again”? There is no doubt, therefore, that in so speaking the Lord signified what death He should die: not that He here meant the death of the cross to be understood, but that the Jews were to deliver Him up to the Gentiles, or, in other words, to the Romans.
For Pilate was a Roman, and had been sent by the Romans into Judea as governor. That, then, this saying of Jesus might be fulfilled, namely, that, being delivered up to them, He should be put to death by the Gentiles, as Jesus had foretold would happen; therefore when Pilate, who was the Roman judge, wished to hand Him back to the Jews, that they might judge Him according to their law, they refused to receive Him, saying, “It is not lawful for us to put any man to death.” And so the saying of Jesus was fulfilled, which He foretold concerning His death, that, being delivered up by the Jews, He should be put to death by the Gentiles: whose crime was less than that of the Jews, who sought by this method to make themselves appear averse to His being put to death, to the end that, not their innocence, but their madness might be made manifest.
Source: Tractates on the Gospel of John (New Advent)