6 In short, that you may know that the words refer, not to the necessity of His dying, but to His power—I speak this for the sake of some who, when they hear “His hour was not yet come,” are determined on believing in fate, and their hearts become infatuated;— that you may know, then, that it was His power of dying, recollect the passion, look at Him crucified. While hanging on the tree, He said, “I thirst.” They, having heard this, offered to Him on the cross vinegar by a sponge on a reed.
He received it, and said, “It is finished;” and, bowing His head, gave up the ghost. You see His power of dying, that He waited for this— until all things should be fulfilled that had been foretold concerning Him— to take place before His death. For the prophet had said, “They gave me gall for my meat; and in my thirst they gave me vinegar to drink.” He waited for all these things to be fulfilled: after they were completed, He said, “It is finished;” and He departed by power, because He came not by necessity.
Hence some wondered more at this His power to die than at His ability to work miracles. For they came to the cross to take the bodies down from the tree, for the Sabbath was drawing near, and the thieves were found still living. The punishment of the cross was so much the harder because it tortured men so long, and all that were crucified were killed by a lingering death. But the thieves, that they might not remain on the tree, were forced to die by having their legs broken, that they might be taken down thence.
The Lord, however, was found to be already dead, and the men marvelled; and they who despised Him when living, so wondered at Him when dead, that some of them said, “Truly this was the Son of God.” Whence also that, brethren, where He says to those that seek Him, “I am He;” and they, going backward, all fell to the ground? Consequently there was in Him supreme power. Nor was He forced to die at an hour; but He waited the hour on which His will might fittingly be done, not that on which necessity might be fulfilled against His will.
Source: Tractates on the Gospel of John (New Advent)