Augustine of Hippo
The city is said to come down from heaven because the grace by which God creates it is heavenly. Therefore He says to it through Isaiah: “I, the Lord, create this” (Isa 45:8).
Indeed, it comes down from heaven from the very beginning, from the time when its citizens, during this age, grow by God’s grace descending from above. They grow through the laver of regeneration in the Holy Spirit, sent down from heaven.
But at the judgment of God, which will be the final judgment through His Son, Jesus Christ, this city, by the gift of God, will appear in such great and new radiance that no trace of oldness will remain. The bodies themselves will pass from the old corruption and mortality to incorruption and immortality.