King James Version of Matthew 3:12
Whose fan is in his hand, and he will thoroughly purge his floor, and gather his wheat into the garner; but he will burn up the chaff with unquenchable fire.
JST
And it is he of whom I shall bear record, whose fan shall be in his hand, and he will thoroughly purge his floor, and gather his wheat into the garner; but in the fullness of his own time will burn up the chaff with unquenchable fire. Thus came John, preaching and baptizing in the river of Jordan; bearing record, that he who was coming after him, had power to baptize with the Holy Ghost and fire.
The LORD directed Joseph to make this verse both longer and more powerful.
The corrected verse is the LORD's way of honoring his forerunner, John the Baptist. JESUS is humble enough to share credit, even though he does 99.99 percent of the work of salvation. John the Baptist was the prophet of the meridian dispensation of the gospel. Joseph Smith is the prophet of this dispensation.
Joseph's version is also more emphatic about John's testimony of JESUS and specifically his power to give someone the HOLY GHOST. HE can also withhold it. No institution controls GOD. If HE wants to give the HOLY GHOST to someone, he will do it, and all the men in the world, and all the devils in hell cannot stop him.
