John Calvin Commentary Isaiah 65:18

"But be ye glad and rejoice for ever in that which I create; for, behold, I create Jerusalem a rejoicing, and her people a joy." — Isaiah 65:18 (ASV)

But rejoice ye and be glad for ever. He exhorts believers to rejoice, as they should, because of such a benefit bestowed by God. This was added for emphasis, because people do not adequately consider God’s other benefits, especially that which is the highest and most excellent of all; for they either disregard them altogether or value them less than they should.

For this reason, believers must be stirred up and urged by such exhortations as these, so that they may not show themselves unthankful or unmindful, or think that it should be lightly passed by, given that, having been redeemed by the hand of Christ, they carry in their hearts the pledge of eternal and heavenly life.

That is why Isaiah shows that believers give due praise for redemption in no other way than by continuing their joy throughout their entire lives and engaging themselves in celebrating the praises of God.

For, lo, I create Jerusalem a rejoicing, and her people a joy. At first, this might seem harsh; but an excellent meaning is found, that the basis for joy in the Church's deliverance will be so great as to remove every cloud of sadness. And, indeed, since even afflictions aid our salvation (Romans 8:28), we have good reason for rejoicing in them.