John Calvin Commentary Isaiah 66:10

"Rejoice ye with Jerusalem, and be glad for her, all ye that love her: rejoice for joy with her, all ye that mourn over her;" — Isaiah 66:10 (ASV)

Rejoice you with Jerusalem. He promises that those who formerly were sad and melancholy will have a joyful condition. For Isaiah has in view not his own age, but the time of the captivity, during which believers continually groaned and, overwhelmed with grief, almost despaired. Therefore, he exhorts and stimulates to joy all believers who are moved by strong affection toward the Church and consider nothing more desirable than her prosperity.

In this way, he instructs them that no one will have a share in so valuable a blessing except those who are prompted by a godly love of the Church and desire to seek her deliverance, even when she is contemptible in the eyes of the world; as the Psalmist says,

For your meek ones love her stones, and will have compassion on her dust. (Psalms 102:14)

Therefore, he adds—

All you that mourn for her; For since in the captivity there was frightful and shocking desolation, and there appeared to be no longer any hope of safety, he arouses believers and encourages them to be of good cheer, or at least to prepare themselves for joy. This exhortation also contains a promise and something more, for a bare promise would not have carried so much weight. But those statements must not be limited exclusively to a single period; for we should abide by the general rule, of which we have often spoken already, namely, that these promises must be extended from the return of the people down to the reign of Christ, and to the full perfection of that reign.