Why did the soldier pierce Jesus' side with the sword? What are the prophesies concerning the sword in the side?
The following passage reveals the answer to both of your questions.
Then the Jews, because it was the day of preparation, so that the bodies would not remain on the cross on the Sabbath (for that Sabbath was a high day), asked Pilate that their legs might be broken, and that they might be taken away. So the soldiers came, and broke the legs of the first man and of the other who was crucified with Him; but coming to Jesus, when they saw that He was already dead, they did not break His legs. But one of the soldiers pierced His side with a spear, and immediately blood and water came out. And he who has seen has testified, and his testimony is true; and he knows that he is telling the truth, so that you also may believe. For these things came to pass to fulfill the Scripture, “NOT A BONE OF HIM SHALL BE BROKEN.” (NASB) John 19:31-36
Pontius Pilate ordered that the legs of those hanging on the crosses be broken. This command was given in order to speed their deaths. After breaking the legs of the two thieves, the soldier decided to not break Jesus’ legs. Instead he took a spear and pierced Jesus’ side. He did not use a sword, since Jesus would have been hanging above the soldier up on the cross. Most likely, it was already obvious that Jesus was dead so the soldier pierced Jesus’ side with the spear in order to verify that Jesus was truly dead. When the soldier did, water and blood came out. The mixture of water and blood occurred for one of several reasons: a) the crassamentum and the serum of the blood had already begun to separate, b) Jesus’ lungs or stomach had filled with water and blood and the spear pierced the corresponding organ, or c) the water came from the pericardium as the result of a coronary rupture.
The prophecy that was fulfilled is given in John 19:36. It is a quote from Psalms 34:20.
He keeps all his bones, Not one of them is broken. (NASB) Ps. 34:20
The prophecy predicted that none of Jesus’ bones would be broken.
It is wonderful to note that the bones of the Passover lamb were not to be broken (Exodus 12:46; Numbers 9:12). The Passover lamb was a type or symbol of the coming Messiah. Jesus died during the Passover festival. Jesus was and is our Passover sacrifice who took away our sins. Praise God! We can have our sins forgiven if we believe that He is our God and want His forgiveness of our sins. The result is that each person who does will spend eternity with God.
1. Merrill C. Tenny. John, The Expositors Bible Commentary. Zondervan Publishing. 1981. vol 9., p. 185.
2. Mark E. Moore. The Chronological Life of Christ. College Press Publishing Co. 1997. vol 2. p. 595.