Never heard Jesus' words "It is finished" explained quite this way before. Courtesy of Illustrated Life of Jesus by Herschel H. Hobb:
This was Jesus' word of completion. It means "it is finished and stands finished." It was a word of full and final completion. Nevermore will Jesus die on a cross. Nevermore will He suffer for the sin of the world. He had made the once-for-all sacrifice. All that was necessary for the Son to do for man's redemption had been accomplished.
The Greek word tetelestai belongs to a family of words used in the legal and commercial life of Jesus' day. One word of this family was used to express the idea of completing a legal deed by dating and signing it. In a very real sense, before the foundation of the world, God had drawn up a deed of redemption for all men who would receive it, but the deed had never been dated or signed. So just before Jesus died He inserted the date of His death, and He signed it in His indelible blood.
The word uttered by Jesus was used in the sense of making full and final payment of a note. Again, in eternity the Son had made out a promissory note of redemption. In this light, one may understand the manner of salvation for those who died before Calvary. They were saved on credit, the credit of the Son of God, looking toward that day when He would pay the note. Therefore, all the Old Testament righteous, like Abraham, had looked forward in joy to Messiah's day.
Now on the cross the promissory note had been paid. So Jesus wrote across it TETELESTAI! Nevermore can payment again be demanded! All who lived before that hour, and since that hour, may in faith receive the redemption God in Christ provided as He died on the cross.
Even more to the point is an ancient usage of this verb where a father sent his son on a mission, saying, "Until you accomplish this for me." It is inferred that upon the successful completion of the mission the son reported, "It is finished."
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Truth is...Three words can pack a powerful punch.