Albert Barnes Commentary Isaiah 66:14

"And ye shall see [it], and your heart shall rejoice, and your bones shall flourish like the tender grass: and the hand of Jehovah shall be known toward his servants; and he will have indignation against his enemies." — Isaiah 66:14 (ASV)

And when you see this – This great addition to the church from the Gentile world.

Your bones shall flourish like an herb – This is an image that is often used in the Scriptures. When the vigor of the body fails, or when it is much afflicted, the bones are said to be feeble or weakened, or to be dried (Psalms 6:2; Psalms 51:8; Psalms 22:14, 17; Psalms 38:3; Lamentations 1:13; Proverbs 14:30; Proverbs 17:22). In a similar way, prosperity, health, and vigor are denoted by making the bones fat (see the notes at Isaiah 58:11; Proverbs 15:20), or by imparting health, marrow, or strength to them (Proverbs 3:8; Proverbs 16:24). The meaning here is that their vigor would be greatly increased.

The hand of the Lord shall be known – That is, it will be seen that he is powerful to defend his people and to punish their enemies.