Jesus is in possession of all necessary knowledge and of how much time is left. Although, to all physical appearances He is completely helpless, the reverse is true and Jesus is totally in command of the whole situation. When God is involved, that which seems certain in the physical realm may not be as it seems. Don’t ever give up and don’t ever lose hope.

The thirst of Jesus proves that He was really and completely human and suffered agony on the cross. This disproved the doctrine of the Gnostics who said that Christ was really a phantom or ghost and did not have a human body because they thought the body to be evil. In fact, it was necessary for Christ to suffer (Heb 2:10, 5:7).

Hebrews 2:10  In bringing many sons to glory, it was fitting that God, for whom and through whom everything exists, should make the author of their salvation perfect through suffering.

Hebrews 5:7  During the days of Jesus’ life on earth, he offered up prayers and petitions with loud cries and tears to the one who could save him from death, and he was heard because of his reverent submission.

The fulfilled scripture to which John refers is (Psa 69:21) but (Psa 22:15) is also relevant. It is important to note that Christ was thirsty so that we will not have to be (Luke 16:19-24).

Psalms 69:21  They put gall in my food and gave me vinegar for my thirst.

Psalms 22:15  My strength is dried up like a potsherd, and my tongue sticks to the roof of my mouth; you lay me in the dust of death.

Luke 16:19-24  “There was a rich man who was dressed in purple and fine linen and lived in luxury every day.  At his gate was laid a beggar named Lazarus, covered with sores and longing to eat what fell from the rich man’s table. Even the dogs came and licked his sores.  “The time came when the beggar died and the angels carried him to Abraham’s side. The rich man also died and was buried.  In hell, where he was in torment, he looked up and saw Abraham far away, with Lazarus by his side.  So he called to him, ‘Father Abraham, have pity on me and send Lazarus to dip the tip of his finger in water and cool my tongue, because I am in agony in this fire.’

These are the only words spoken by Jesus which seem to be a complaint about His circumstances. When He was accused and when He was mistreated by the scourging, beatings, thorns, and the mockings, He did not say anything (Isa 53:7) and yet He does so here, and He does it for 2 reasons. First, as John points out, it was necessary to fulfill scripture; and second, He is soon going to need to say something which must be heard and understood, and He needs the drink to moisten His parched mouth so His words will be understood.

Isaiah 53:7  He was oppressed and afflicted, yet he did not open his mouth; he was led like a lamb to the slaughter, and as a sheep before her shearers is silent, so he did not open his mouth.