To my mind the character of "Confession" - which comes from the fact that Augustine regularly speaks to God through prayer or praise - is mostly an excuse to hide the vanity and self-centeredness that has behind it all autobiography. It is difficult to question the deep faith of Augustine - either through what he describes his life in this book or through what history has left us of his actions, but he said -even that despite its progress in faith and virtue it still feels driven by his instincts and desires.
It is this tension between reason and faith that is present throughout the book. It illustrates the difficulty to reconcile philosophy and religion by definition, religion deals with what comes out of the domain of knowledge, and is purely the domain of subjectivity. Here Augustine tries to show vis-à-vis objectivity of his own subjectivity, with a result that does not appear to me very convincing: it indicates in fact that the very fact that science and knowledge have limits are evidence of the existence and omnipotence of God, beyond these limits. It also provides a description of the nature of God, starting from what he imagined when he was a follower of the Manichean, but it happens in the end as the vague description of the nature of God is that of official the Catholic Church today. How could it be otherwise? If he had been completely rationally define the nature of God, he would have withdrawn from the realm of faith, and would have placed in the field of knowledge. And so there would be no question of faith: we no longer believe, we know. And while the man is free to believe or not, deny knowledge is not possible.
It is therefore only in its purely rational purpose - allow others to put in its place by delivering his thoughts and his views - Augustine is compelling. And his whole life is a testimony to the voltage at which every man is subject between his instincts and reason, and the fact that this tension is dynamic whatever the progress of reason, and possibly in parallel of faith, new desires pulling in the opposite direction appear.
And the undeniable link updates Augustine between reason and faith is that faith directs reason, it is a kind of rudder for what would ultimately just a boat adrift, without purpose. In a sense, faith gives a purpose to reason, because it invites to progress. The direction in which it calls for progress depends she that wherein one has faith. As thinking like Augustine believed that the end of this course is the one Truth, well, it's a matter of belief!