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I am not a medical person, so others may explain better than I can. I understand from reading that the separation of haemoglobin (red) from the serum (clear) meant the body was already lifeless and showing early signs of deterioration.
This is recorded so we may know that Jesus was truly dead, even before the spear thrust that by itself would have killed him.
There are those who insist Jesus only "swooned" on the cross and later revived in the tomb, not died and rose from the dead. This medical information clearly refutes such a notion as Jesus falling into a temporary coma, then "getting better" again within a few days of his torture and crucifixion and returning to his disciples.
Although the Roman soldiers intended, by the piercing, to determine if Our Lord and Savior, Jesus Christ was still alive or dead, little did they know that they were unwittingly helping to fulfill the program of our Heavenly Father, the Almighty God for Christ's life - that is, to open more room for blood to gush out for our cleansing from sin, being in right standing with God, weapon of victory over Satan and his collaborators, testimony etc. The assurance of death of Jesus to the soldiers through this process helps to shut the mouth up of those satanic agents who propagate the wicked and false idea that Jesus merely fainted and never died.
Had Jesus not died, true believers could have been most miserable. We thank God that Jesus truly died and rose from death. This guarantees our future resurrection to be with the Lord for ever.
Pastor Alex Nwanne
I also believe that since there is no blood in heaven, the water was a symbol that Jesus was the Son of man, and the Son of God.