You know "The Fire Sermon" section of T.S. Eliot's super-major-important poem, "The Wasteland"? Augustine may as well be the one giving that sermon, because he's all over it… along with the Buddha, because the East always has to meet the West in Eliot, right Modernists?
Take a looksee at this Augustine-focused section of Eliot:
To Carthage then I came
Burning burning burning burning
O Lord Thou pluckest me out
O Lord Thou pluckest
We think it's pretty safe to say that if someone as smart as T.S. Eliot saw fire as the main motif of the Confessions, then it probably is.
Fire as Lust
One of the most important lines in the book (partially due to Eliot) occurs at the very beginning of Book III:
I went to Carthage, where I found myself in the midst of a hissing cauldron of lust. (III.1.1)
This is when we know that Augustine's sinning is really getting serious, because now he's a young adult, out on his own in the world. In this line, fire is associated with passion, lust, and sex, all of which are good n' plenty in old Carthage Town.
Fire can refer to Augustine's penchant for other sins as well:
I was much attracted by the theatre, because the plays reflected my own unhappy plight and were tinder to my fire. (III.2.1)
Poor Augustine is feeding his own demise, in typical vicious-cycle form.
Fire as Holy Passion
But fire can refer to passion of a different kind, and in the Confessions it eventually does: we're talking about that wholesome, burning passion for God. Fire goes from something that Augustine needed to be saved from to something that saves him. See, fire can be very positive when it's put to a spiritual use rather than a lustful one. Ladies and gentlemen of the jury, Exhibit A:
How they set me on fire with love of you! I was burning to echo them to all the world. (IX.4.3)
Before, fire seemed to represent destruction—a destruction that would eventually burn Augustine up unless he was saved. Here, fire seems to represent something like vitality, as if a light were being turned on (speaking of which, see the Light and Dark motif). And it's this God-lovin' fire that you want to spread:
For when large numbers of people share their joy in common the happiness of each is greater, because each adds fuel to the other's flame. (VIII.4.2)
You're probably asking yourself, "Why does Augustine pull a complete 180 on this whole fire business?" Well, we might want to think about the idea of transformation, which is pretty important in the Confessions. The Confessions are all about Augustine's transformation from a sinner to a devout bishop, so it's possible that fire, which really represents some kind of zeal, changes in nature when it is being used for God instead of for earthly things. Shazam.