John Gill 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, and be glad with her
The church; she bringing forth so many spiritual children to Christ; just as, when a woman is delivered of a child, her friends and neighbours congratulate her upon it, and rejoice with her on that account; as Elisabeth's neighbours and relations did, at the birth of John the Baptist, (Luke 1:57Luke 1:58) , so the church's friends here are called together to rejoice with her, at the numerous birth and conversion of souls in her, than which nothing is more joyful to the saints; see (Luke 15:6Luke 15:9Luke 15:23Luke 15:24) (Acts 15:3) :

all you that love her ;
wish her well, and pray for her peace and prosperity; all that love God love his church, the habitation of his house, the place where his honour dwells; all that love Christ love her who is his spouse and bride, and purchased by his blood; all that love the word and ordinances love the church where they are administered:

rejoice for joy with her, all you that mourn for her ;
that had mourned for her, when things went ill with her; these are they that mourn in Zion, and for Zion, because of the sins of her professors; corruptions in doctrine, discipline, and worship; declensions in grace; want of love to one another, and few instances of conversion: but now things being the reverse, and it going well with her, they are called upon to rejoice exceedingly with her; for such is the sympathizing spirit of the saints, that they rejoice with them that rejoice, and weep with them that weep, (Romans 12:15) (1 Corinthians 12:26) .