D&C 77

The Prophet Joseph Smith was asked - 

"...What are we to understand by the little book which was eaten by John, as mentioned in the 10th chapter of Revelation?"  (Doctrine and Covenants 77:14)

One answer was - 

"We are to understand that it was a mission, and an ordinance, for him to gather the tribes of Israel..."  (Doctrine and Covenants 77:14)

The angel explained to John that the work of the gathering of Israel would "sweet as honey" and "bitter".  Obviously a challenge.