Blood and Water

How do we know for sure that Jesus died on the cross?
In the human body, water doesn’t separate from blood until circulation stops, at which point it accumulates in the heart and lungs. Since both "blood and water came out" (John 19:34) when "one of the soldiers pierced His side with a spear" (John 19:34), circulation had stopped, so Jesus was dead.

But John 19:34 says that the spear strike pierced His "side," which is below his heart and lungs?
Jesus' body was up on a cross, with the soldier below. So the spear strike pierced His side at an upward angle and punctured His vital organ. Note that even if Jesus had been perfectly healthy at this time, this spear strike on its own would have killed Him.

Who had returned to the cross after taking Mary to his home?
John (see His mother): "he who has seen has testified" (John 19:35).

Why else is Jesus having been pierced significant?
It fulfilled another Old Testament prophecy: "And I will pour on the house of David and on the inhabitants of Jerusalem the Spirit of grace and supplication; then they will look on Me whom they pierced..." (Zachariah 12:10).

Were any other prophecies fulfilled?
Prophecies about Jesus' birth, life, death and resurrection are found throughout the Old Testament. One of them, given through David a thousand years before Jesus was crucified on the cross, describes the details of His crucifixion long before that method of execution was even invented by the Romans, as well as His cry from the cross (see Psalm 22). Another prophecy, given through Isaiah about 700 years before Jesus' sacrifice, explains the reason for His sacrifice and foretells His death with criminals ("with the wicked") and His burial by rich men ("with the rich at His death"), which Joseph of Arimathea and Nicodemus were (see Isaiah 53 and the next page).