I think its a wonderful quote from Augustine, which I first heard through reading C.S Lewis.
I don't think its true that people will believe whatever they want to - though this temptation always exists and I think this is increasingly what is happening with the "do-it-your-self-" spiritualities gaining ground in popular culture, this fact being among its appeal. We have certainly come to resent the imperatives of revealed faith.
This quote certainly stikes a chord with me for the days when I practiced ecclectic spirituality that had no such need of the institutions of religion- when I simply followed my own lead and found only my own footsteps in the pages of the Gospels.
In my experience, the meaning of this saying is this:
That if one believes in Christ, truly believes in him, then one can not be left unchanged, one can not use him to justify their own self simply as it is. He will make demands which we don't wish meet and he won't come to us quite as we would like. These will be diverse and every one will struggle with their own particular demands.
For me, the evidence that I had truly found Christ, or that he had come to me, was found in something like this- the turmoil of being pulled apart inside, being faced with demands which I did not (and still do not) wish to live up to.
I often feel like my journey with Christ is like two men who have come upon an old and decayed brick wall and one has went to work building it up again, while the other (Christ) is swift at work to give time a little a nudge and finish it off. I lay a brick down, turn to get the next, and turn back to see that the Lord has removed it.
This might be a poor or strange illustration- but I'm just finishing up this bottle of wine that I have here....maybe other Christians understand what I am saying.