The idea of recurring cycles of existence is completely at odds with any conception of heaven as a place of eternal bliss. It would be cruel to pull someone from heaven and thrust them back into the sufferings and miseries of earth. To take from them the joys of the infinite radiance of God, and return to a life of sin and the possibility of hell is the worst thing one could do.
Again speaking pastorally, Augustine notes the devastation this idea has on one's hope:
For who would not be more remiss and lukewarm in his love for a person whom he thinks he shall be forced to abandon, and whose truth and wisdom he shall come to hate; and this, too, after he has quite attained to the utmost and most blissful knowledge of Him that he is capable of? Can any one be faithful in his love, even to a human friend, if he knows that he is destined to become his enemy? God forbid that there be any truth in an opinion which threatens us with a real misery that is never to end, but is often and endlessly to be interrupted by intervals of fallacious happiness. For what happiness can be more fallacious and false than that in whose blaze of truth we yet remain ignorant that we shall be miserable, or in whose most secure citadel we yetfear that we shall be so? For if, on the one hand, we are to be ignorant of coming calamity, then our present misery is not so short-sighted for it is assured of coming bliss. If, on the other hand, the disaster that threatens is not concealed from us in the world to come, then the time of misery which is to be at last exchanged for a state of blessedness, is spent by the soul more happily than its time of happiness, which is to end in a return to misery. And thus our expectation of unhappiness is happy, but of happiness unhappy. And therefore, as we here suffer present ills, and hereafter fear ills that are imminent, it were truer to say that we shall always be miserable than that we can some time be happy.Fortunately, the truth stands against this error, and Christ has rescued us for real and from all danger of hell--in this life and for all eternity.
Having dismissed the idea of reincarnation and cyclical existence (which would include most modern scientific worldviews), Augustine briefly mentions the debate over whether the number of believers is set by God, or changes over time. He does not respond to this debate here, and only points out (again, pastorally) that in either case there must have been a point at which man was created. If man were eternal, we could never have such a debate. And it is to this creation which we are now to turn.