Why was jesus cut with a spear?

Answer 1

John is the only gospel writer to mention this event.

There are two reasons for this. First, John was the only disciple not to desert Jesus at the crucifixion and so he was the only real close-by eyewitness from the group of disciples (although others were there like Mary his mother, Mary Magdalene and so on). Therefore, as John was a witness he included it in his gospel narrative.

Secondly, John makes clear the reason for writing his gospel - so that we can believe Jesus is divine. Therefore John records the moment that Jesus was stabbed by a spear to show that he really was dead and to dispel any silly myths that Jesus had only fainted and had subsequently come round in the cool of the tomb. John included it to show Jesus was dead. Period. And also that when the resurrection happened it was therefore categorically the greatest miracle the world had ever seen, or will see.

The reason why the soldier stabbed Jesus in the first place was to ensure he was dead. Roman soldiers who carried out executions were severely punished if they botched them or the victim survived. In the case of the two thieves, the soldiers broke their legs so that they could not lift themselves up by their feet in order to breathe more easily. This procedure hastened their death. However, when they came to Jesus they saw he was already dead, and therefore didn't break his legs, but instead thrust the spear into his side to ensure he was dead. John, as a nearby eyewitness, records that 'blood and water' came oout of the wound - ie congealed clot and serum, proving that Jesus' heart had stopped beating long enough before to allow the blood to clot in his veins.