How would 26 year old Joseph Smith know to use the word “unjust” rather than “damnation” in D&C 76:17?

In Doctrine and Covenants 76:17, Joseph Smith refers to John 5:29:

And shall come forth; they who have done good, in the resurrection of the just; and they who have done evil, in the resurrection of the unjust. (emphasis added)

The King James Version of this scripture reads:

And shall come forth; they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of damnation. (emphasis added)

Why the difference between the two? Elder Quentin L. Cook spoke in general conference of this inspired translation:

In the 17th verse of section 76, he was inspired to use the word unjust instead of damnation that was used in the Gospel of John.
It is interesting that 45 years later an Anglican church leader and academically credentialed classical scholar, Frederic W. Farrar, who wrote The Life of Christ, asserted that the definition of damnation in the King James Version of the Bible was the result of translation errors from Hebrew and Greek to English.
– Elder Quentin L. Cook – Conversion to the Will of God

How would Joseph Smith have known of this translation error?

See:

Elder Quentin L. Cook – Conversion to the Will of God

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