9 Hear, thou fool: “His hour was not yet come;” not the hour in which He should be forced to die, but that in which He would deign to be put to death. For Himself knew when He should die: He considered all things that were foretold of Him, and awaited all to be finished that was foretold to be before His suffering; that when all should be fulfilled, then should come His suffering in set order, not by fatal necessity. In short, hear that you may prove. Among the rest that was prophesied of Him, it is also written: “They gave me gall for meat, and in my thirst they gave me vinegar to drink.” How this happened, we know from the Gospel.
First, they gave Him gall; He received it, tasted it, and spat it out. Thereafter, as He hung on the cross, that all that was foretold might be fulfilled, He said, “I thirst.” They took a sponge filled with vinegar, bound it to a reed, and put it to His mouth; He received it, and said, “It is finished.” What did that mean? All things which were prophesied before my death are completed, then what do I here any longer? In a word, when He said “It is finished, He bowed His head, and gave up the ghost.”
Did the thieves, who were nailed beside Him, expire when they would? They were held by the bonds of flesh, for they were not the creators of the flesh; fixed by nails, they were a long time tormented, because they had not lordship over their weakness. The Lord, however, when He would, took flesh in a virgin's womb: came forth to men when He would; lived among men so long as He would; and when He would He quitted the flesh. This is the part of power, not of necessity. This hour, then, He awaited; not the fated, but the fitting and voluntary hour; that all might first be fulfilled which behooved to be fulfilled before His decease.
How could he have been under necessity of fate, when He said in another place, “I have power to lay down my life, and I have power to take it again: no man takes it from me, but I lay it down of myself and take it again?” He showed this power when the Jews sought Him. “Whom do you seek?” says He. “Jesus,” said they. And He answered, “I am He.” When they heard this voice, “they went back and fell to the ground.”
Source: Tractates on the Gospel of John (New Advent)