"After this, Jesus knowing that all things were now accomplished, that the scripture might be fulfilled, saith, I thirst." (John 19:28)
Because Jesus thirsted on the Cross, He sent the message to the whole universe and for all time that He had completed the work He came to do:
"Jesus saith unto them, My meat is to do the will of him that sent me, and to finish his work." John 4: 34)
What was the work? Jesus explained to Nicodemus:
"16For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. 17For God sent not his Son into the world to condemn the world; but that the world through him might be saved. 18He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God." (John 3: 16-18)
"For I came down from heaven, not to do mine own will, but the will of him that sent me." (John 6: 38)
Paul explained the fullness of what Jesus did at the Cross:
"Be it known unto you therefore, men and brethren, that through this man is preached unto you the forgiveness of sins: 39And by him all that believe are justified from all things, from which ye could not be justified by the law of Moses." (Acts 13: 38-39)
What else did Jesus' thirst signify? That the fire of God's wrath had been loosed upon Jesus, and God's wrath fully satisfied.
"Much more then, being now justified by his blood, we shall be saved from wrath through him." (Romans 5: 9)
Jesus thirsted so that we never have to:
"And Jesus said unto them, I am the bread of life: he that cometh to me shall never hunger; and he that believeth on me shall never thirst." (John 6: 35)
"He that believeth on me, as the scripture hath said, out of his belly shall flow rivers of living water." (John 7:38)
"They shall hunger no more, neither thirst any more; neither shall the sun light on them, nor any heat." (Revelation 7: 16)
Jesus thirsted on the Cross, so that today we have His Spirit, and living waters flow in and through us. We will never thirst again because Jesus has given us His life, and that more abundantly (John 10: 10).