After the fall of Adam, man has since been polluted with a sinful nature. In God's plan of redemption to redeem fallen man...Jesus had to become a man... to be the atoning sacrificial lamb offered back to God for the sins of humanity. Christ's humanity allowed God's redemptive work to be given to fallen man. The Bible describes Christ as atoning the Lamb of God...
1 Peter 1:19-20...But with the precious blood of Christ, as of a lamb without blemish and without spot: Who verily was foreordained before the foundation of the world,
Rev 13:8...whose names are not written in the book of life of the Lamb slain from the foundation of the world.
In Saint John's day when this Apostle of Christ wrote his Epistles, he was contending with a pagan religion call Gnosticism. Gnostic's denied the fact that Christ was a human being but they accepted the fact that Christ was very God. Even Gnostic religion couldn't deny the fact that Jesus had performed so many miracles at that day and hour. Christ was renowned in many countries in Asia minor...as the promised Messiah who miraculously died and rose from the grave. The truth about the Lamb of God slain from the foundation of the world was spreading throughout all of Europe through the preaching of the Gospel message.
As the Prophets declared...God came down from Heaven in the form of a man. Christ came in the form of a seed and was placed within the womb of the virgin Mary by the Spirit of Holiness...according to God's promises. Christ the promised "seed" was the only seed of a woman in the history of humanity. This miracle seed formed the Christ child in the womb of a virgin. All men "except" Christ are made of the seed of a man. Christ (God in the flesh) was made of the seed of woman. Think about that miracle! Glory! Christ was made of the seed of a woman!
Gen 3:15...And I will put enmity between thee and the woman, and between thy seed and "her seed;" it shall bruise thy head,
Gal 4:4-5...But when the fulness of the time was come, God sent forth his Son, made of a woman
Christ who was God in the beginning was also... the "Word of God" as they are one in the same. Christ, the Word of God, became flesh and dwelt among us.
John 1:14...And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,)
When Christ as the Lamb of God slain from the foundation of the world hung on the cross, we can find these distinct words that prove that Christ was a man. Notice the physical anatomy of the man Christ! The "water and blood" prove the human anatomy of Christ. The water and blood prove that Christ was a human being just like you and I. Christ's humanity proved the truth about God's Promises. Christ's humanity was also proven in the records of Saint John's Gospel.
John 19:34-36...But one of the soldiers with a spear pierced his side, and forthwith came there out blood and water. And he that saw it bare record, and his record is true: and he knoweth that he saith true, that ye might believe. For these things were done, that the scripture should be fulfilled, A bone of him shall not be broken.
This "very humanity of Christ" is what John was refuting with the Gnostic religion...in His First Epistle. Note John's description of the very make-up of the human being which consists of about 80-90% water. This description of water and blood in the following verse simply defines the human make-up of Christ in rebuttal against Gnosticism.
1 John 5:6... This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood.
Gnosticism denied the fact of Christ's humanity. This very denial made by the Gnostics is what John was "reproving" when he spoke of, "the sin unto death." Anyone knows that God in His mercy is compassionate and loving to forgive...BUT THE DENIAL OF THE HUMANITY OF CHRIST WAS UNPARDONABLE AND WAS CALLED A SIN UNTO DEATH THAT COULD NOT BE PRAYED FOR.
1 John 5:16... If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. "There is a sin unto death: I do not say that he shall pray for it".
The mysterious... "sin unto death" taught in John's First Epistle was simply God's Judgment against the Gnostics denial of the humanity of Christ. The pagan concept of Gnosticism was tainting the true Gospel message that Christians had been taught by the Apostle John...after the death and resurrection of Christ.