Finally, we get to a discussion of the end of the City of God. This will be an "eternal" end, where "eternal" simply means the length of time which does not stop. The highest parts of God's creation (angels and men) were created to be eternal and to live in fellowship with Him forever. We were to feed off of His joy and delight through communion in His presence for all time. However, God also made us mutable, changeable, and with a free will. And using that will, we rejected God and tried to find our happiness and our joy in ourselves. "But, as with the angels, if human nature should choose to fall away from God, misery proportionate to the offense was bound to follow. Here, too, God foresaw the fall, the disregard of His law, the desertion from Good, yet He left man's free choice unchecked..."
But why would God do this? "..because He also foresaw to what good He would turn man's evil. And, in fact, out of this mortal race of men, justly doomed by their own deserts, God gathers, by His grace, so numerous a people that out of them He fills the places and restores the ranks emptied by the fallen angels."
We are lost by the exercise of our own wills, but restored to God by His grace. And so the City of God will not want for its full complement of citizens.
All of creation, both the City of God and the city of man, are governed by the will of God, which has ordained what will be and who will do it. This will is unchangeable, and that is our safety and security (passages in Scripture which seem to suggest that God changes His mind are spoken to us in such a way that we might understand--in fact we are the ones who are changing, not God).
So God has promised to lead His people into the City of God, and these promises are unshakable.