This quality of rest is what defines the City of God as it basks in the light of God's Presence. Likewise, a lack of rest is what defines the city of man by contrast:
...we may say, the one dwelling in the heaven of heavens, the other cast thence, and raging through the lower regions of the air; the one tranquil in the brightness of piety, the other tempest-tossed with beclouding desires; the one, at God’s pleasure, tenderly succoring, justly avenging,—the other, set on by its own pride, boiling with the lust of subduing and hurting; the one the minister of God’s goodness to the utmost of their good pleasure, the other held in by God’s power from doing the harm it would; the former laughing at the latter when it does good unwillingly by its persecutions, the latter envying the former when it gathers in its pilgrims.This may (and Augustine stresses that this may not have been the author's intention) be what the creation story means when it discusses separating light from darkness-- that is, it means splitting the angels into the two cities.
Whatever it means, we gain much by the contemplation of the one City, full of light and life and rest, and the other, full of turmoil, pain, and death.
Some people argue that God did not create everything--namely "water," since it is not mentioned in the creation story. Augustine points out that it is included implicitly in the phrase "heavens and earth." Nor should we get too caught up in that sort of argument, as some are wont to do. Instead, we should be content that we have enough information about the heavenly City of God to begin to delve into its distinctions from the city of man.