In the context of this roughly sketched answer, Augustine notes a deeper meaning of the word "beginning." God himself (in the form of Christ, who is the living "Word" of God) is the "beginning," not in the sense that he was there "first" (remember, God is eternal and has nothing to do with time) but in the sense that he is the "fixed point" to which we can return. "The Word" is first in the sense that he is the first cause, the unmoving point that is the source of all things. This reading of the "beginning" as the Word (Christ) allows Augustine to get around the apparently temporal implications of the "beginning" in Genesis.
Another way of stating this same interpretation is to refer to Christ (who is the "beginning") as "wisdom." Christ, for Augustine (and for all Christians), is the route by which one can seek the wisdom of God. Hence, Augustine can write here: "Wisdom is the beginning, and in the beginning you made heaven and earth." Again, this is a profoundly spiritual reading of the words used in Genesis. We are no longer talking about a temporal beginning at all, but simply about the context of eternal wisdom (accessible to us through Christ) in which God eternally "makes" the world.
Such a reading of Genesis also allows Augustine to respond to a criticism made by the Neoplatonist Porphyry (the primary disciple of Plotinus). Porphyry claimed that the creation was impossible, because there would have had to be a moment when God decided to create. In other words, the will of God (which is by definition unchanging) would have had to change.
Augustine can now reply that this is a misconception based on the failure to recognize the eternal, constant sense of the word "creation." God did not create the universe at a given time, because for God there is no time. The act of creation is both instantaneous and eternal. Since time is a feature only of the created world (not of God), there couldn't have been any time before God created the universe. Augustine puts this in a number of ways: "There was no 'then' when there was no time," or, "It is not in time that you [God] precede all times. Otherwise you would not precede all times." Again, God is "first" only in the sense of being the eternal cause of all creation. He wasn't "doing" anything before he created the world (a common Manichee challenge), because there was no "before."
[XI.17-41] Augustine now begins to consider time itself. He has argued that time has nothing to do with God himself (thus clearing up the apparent temporality of the creation act), but the creation in which we live still seems to exist in time. Following Aristotle, Augustine notes that everyone thinks they know what time is, at least until they are asked.
Past, present, and future seem to be the defining elements of time. Augustine begins, then, by noting that time depends on things passing away (past), things existing (present), and things arriving (future). Already, Augustine is ready to hint at a significant point: if time is defined by things arriving, remaining for a moment, and passing away, then time seems to depend utterly on a movement toward non-being. As Augustine quickly concludes, "indeed we cannot truly say that time exists except in the sense that it tends toward non-existence."