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Zech.12
[10] And I will pour upon the house of David, and upon the inhabitants of Jerusalem, the spirit of grace and of supplications: and they shall look upon me whom they have pierced, and they shall mourn for him, as one mourneth for his only son, and shall be in bitterness for him, as one that is in bitterness for his firstborn.
So we have the God of the OT speaking about a future time when He will be "pierced", mourned for as an "only son" and shall be in bitterness as for his "firstborn"
Now who is this? Let's find out. Let's see if all that God said would happen to Him actually did.
John.19
[34] But one of the soldiers with a spear pierced his side, and forthwith came there out blood and water.
[37] And again another scripture saith, They shall look on him whom they pierced. (Sounds familiar?)
Rev.1
[7] Behold, he cometh with clouds; and every eye shall see him, and they also which pierced him: and all kindreds of the earth shall wail because of him. Even so, Amen.
So it was Jesus Christ that was "pierced". Let's read more;
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,) full of grace and truth.
John.3
[16] For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
It was Jesus Christ that was called the, "only begotten Son". Here is more;
Col.1
[15] Who is the image of the invisible God, the firstborn of every creature:
Rom.8
[29] For whom he did foreknow, he also did predestinate to be conformed to the image of his Son, that he might be the firstborn among many brethren.
And finally, it was Jesus Christ that was called the "firstborn".
Now the God of the OT said ALL of these things would happen to HIM. And as we just read, each and everything He said would happen did. Now who did ALL of these things happen to? Jesus Christ!
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