Albert Barnes 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 - The idea presented in this verse is that it is the duty of all who love Zion to sympathize with her joys. It is one evidence of piety to rejoice in her joy; and those who have no true joy when God pours down His Spirit and produces changes in a revival of religion as sudden and transforming as if the earth were suddenly to pass from the desolation of winter to the verdure and bloom of summer, or when the gospel makes rapid advances in the pagan world, have no true evidence that they love God or His cause.

Such scenes awaken deep interest in the hearts of angels and in the heart of God the Savior; and those who love that God and Savior will rejoice in such scenes, and will mingle their joys and thanksgivings with the joys and thanksgivings of those who are thus converted and saved.

All you who mourn for her - those who sympathize with her sorrows and mourn over her desolations.