"Consummatum est ~ It is finished"

"After this Jesus, knowing that all was now finished, said (to fulfill the scripture), "I thirst."  A bowl full of vinegar stood there; so they put a sponge full of the vinegar on hyssop and held it to his mouth.  When Jesus had received the vinegar, he said, "It is finished"; and he bowed his head and gave up his spirit". (St. John ,19, 28-30)

Commentary of St. Thomas Aquinas on the Gospel of St. John: 

In regard to the first he says, After this, after the things that had just been mentioned, Jesus, knowing that all was now finished, that is, all that the law and the prophets had foretold about him had now been accomplished: "Everything written about me in the law of Moses and the prophets and the psalms must be fulfilled" (Lk 24:44); "I have seen the end of every consummation" [Ps 119:96].

But because another thing foretold in scripture had to be done, the Evangelist adds that Jesus said (to fulfill the scripture), I thirst. First, we see the words spoken by Christ; then, how his desire could be satisfied; and finally, he is given the vinegar...

By saying, I thirst, he showed that his death was real, and not just imaginary. It also indicated his intense desire for the salvation of the human race: "God our Savior, who desires all men to be saved" (1 Tim 2:4); "For the Son of man came to seek and to save the lost" (Lk 19:10). Indeed, we express our intense desires in terms of thirst: "My soul thirsts for God" (Ps 42:3).

 This desire could be satisfied because a bowl full of vinegar stood there.This bowl signified the Jewish synagogue, in which the wine of the Patriarchs and Prophets had degenerated into vinegar, that is, into the malice and severity of the chief priests.

Christ is given the vinegar, for they put a sponge full of vinegar on hyssop and held it to his mouth. There is a question on the literal meaning. How could they put the sponge to Christ's mouth, since he was hanging high off the ground? This is answered by Matthew (27:48), who says that the sponge was put on a reed. Or, according to others, it was put on hyssop, which was long, and this is what Matthew called a reed.

As for the mystical sense, these three things signify the three evils that were present in the Jews: the vinegar signifies their ill‑will; the sponge, full of crooked hiding places, signifies their craftiness; and the bitterness of the hyssop stands for their malice. Or, the hyssop represents the humility of Christ, for the hyssop is a bush used for purification, and our hearts are purified especially by humility: "Sprinkle me with hyssop and I will be cleansed" [Ps 51:7].

The final fulfillment is mentioned when the Evangelist says, When Jesus had received the vinegar, he said, It is finished. This can be understood to refer to the fulfillment accomplished by Christ by dying: "For it was fitting that the author of our salvation be fulfilled by glory through his passion" [Heb 2:10]. Or, it can be understood to refer to the fulfillment or accomplishment of our sanctification, which was brought about by his passion and cross: "For by a single offering he has perfected for all time those who are sanctified" (Heb 10:14). It can also refer to the fulfillment of the scriptures: "Everything that is written of the Son of man by the prophets will be accomplished" (Lk 18:31).

Then the Evangelist describes the death of Christ. First, he mentions the cause of his death, he bowed his head. We should not think that because he gave up his spirit, he bowed his head; rather, because he bowed his head, he gave up his spirit, for the bowing of his head indicated that he died out of obedience: "He became obedient unto death" (Phil 2:8).