It is finished. John 19:28-37

John 19 NLT
The Death of Jesus
28 Jesus knew that His mission was now finished, and to fulfill Scripture He said, “I am thirsty.” 29 A jar of sour wine was sitting there, so they soaked a sponge in it, put it on a hyssop branch, and held it up to His lips. 30 When Jesus had tasted it, He said, “It is finished!” 
31 It was the day of preparation, and the Jewish leaders didn’t want the bodies hanging there the next day, which was the Sabbath (and a very special Sabbath, because it was the Passover). So they asked Pilate to hasten their deaths by ordering that their legs be broken. Then their bodies could be taken down. 32 So the soldiers came and broke the legs of the two men crucified with Jesus. 33 But when they came to Jesus, they saw that He was already dead, so they didn’t break His legs. 34 One of the soldiers, however, pierced His side with a spear, and immediately blood and water flowed out. 35 (This report is from an eyewitness giving an accurate account. He speaks the truth so that you also can believe.) 36 These things happened in fulfillment of the Scriptures that say, “Not one of His bones will be broken,” 37 and “They will look on the one they pierced.”

"It is finished!" Three words that mean so much. He finished what He was sent to do, fulfill the final sacrifice for mankind's sin. No longer was there a need for sacrificial rituals to cleanse us from our sins, our faith in Christ is all we need to be forgiven. He was the perfect, unblemished lamb who's death cleanses us. 

There are medical explanations for the blood and water that flowed from Jesus' side but I don't think the medical explanation is what God wanted us to see by the piercing of Jesus. The following is an excerpt from Matthew Henry's Commentary in Bible Hub: 

"He died in less time than persons crucified commonly did. It showed that He had laid down His life of Himself. The spear broke up the very fountains of life; no human body could survive such a wound. But its being so solemnly attested, shows there was something peculiar in it. The blood and water that flowed out, signified those two great benefits which all believers partake of through Christ, justification and sanctification; blood for atonement, water for purification. They both flow from the pierced side of our Redeemer."

I love that! "blood for atonement, water for purification." I believe that's the point God wanted to make! Washed by the blood, purified by baptism. Have you been baptized since you came to believe? If not, you should be. Now, don't misunderstand me, you do not HAVE to be baptized to enter Heaven, the thief on the cross next to Jesus wasn't but we are told in Luke 23:43 And Jesus replied, “I assure you, today you will be with Me in paradise.”

Blessings, Cecilia