The apostle John, who was the only disciple near the cross when Jesus was crucified, was also the only gospel writer who reported these final details of Christ’s death. Perhaps John made sure to include his eyewitness testimony of the grisly details because of the Gnostic heresy that was already at large by the time of John’s writing.
The Gnostics (From the Greek word gnōsis, meaning “knowledge”) believed that they had obtained mystical or secret knowledge of the divine. They saw the material world as inherently evil and only the spiritual as good. Therefore, they rejected the idea that Jesus had actually come in the flesh. And as a result, they rejected that Jesus had truly died on the cross. In their view, “it only appeared” as if Jesus had died (The Gnostic heresy clearly affected Islam’s view of the crucifixion as the Quran uses an almost identical description).
John’s gospel was especially concerned with reporting the diligence of the Roman executioners in confirming Christ’s physical death. He reported the soldier’s spear piercing Christ’s side and the mixture of “blood and water” pouring out. Although John had no awareness of modern medical knowledge, physicians today have noted that a piercing of the pericardium, the fluid-filled membrane surrounding the heart, would’ve resulted in the sight that John witnessed. The blood and water was a sure sign of Christ’s death.
John followed his reporting of the blood and water with a strong declaration that reminds one of a courtroom witness taking an oath to speak the truth. He said, “And he who has seen has testified, and his testimony is true; and he knows that he is telling the truth, so that you may believe” (John 19:35).
John wanted to make sure that everyone knew that he had witnessed Christ’s physical death. For if Jesus didn’t die, there would be no redemption, no payment for our sin. And if Jesus didn’t die, there would be no need for his resurrection.
But Jesus did die and He was raised. That is the truth that John reported. And that is the truth that we believe.