The very fact that we see the holy family up the street from Brian's birth establishs that Brian is definately not Jesus.
The followers we see in the film are not Jesus' followers, so where does "the general message is that people who followed Jesus when he was alive the first time around didn't think for themselves" come from?
There is no suggestion that Brian's followers "just wanted a religion, and so hailed him as a messiah". Brian's followers already had a religion - they were Jewish. And 1st century Jews interpreted the prophecies of their holy book to refer specifically to a Jewish leader who would free them from Roman occupation, so yes, Jews at that time were looking for a messiah. Messiah, in a Jewish context refers to the anticipated king of the Davidic line prophesied by Isaiah, who will rule the Jewish people. If people were actively seeking their leader who's to say they wouldn't interpret every one of his actions as a sign?
"All the film's saying is that Christianity is nothing more than a mistaken interpretation of something witnessed first hand and it generally made a mockery of faith." - No, that's not all the film is saying. In fact is does not mention Christianity at all, as Christianity had not been invented at the time that the film was set. It does not mock faith, it mocks BLIND faith.
"It painted the followers - and, by association, Jesus' followers - as complete dunderheads and plainly suggested that not only could they not think rationally, but that they also promoted the religion to the extent that millions of humans would spend the next two thousand years chasing an illusion. " - It does nothing of the sort. It depicts Brian's followers as a group of people desperate for answers to massive questions, and grasping at straws when they believe they have found their leader. Again, Jesus' followers are not mentioned, but lets not forget that at that time Jesus only had 12 "official" followers, one of whom sold him to the Romans so that he could be nailed to a plank and another one who denied knowing him at all when it came to crunch time.
You have made big assumptions in your argument, and have flitted between Brian and Jesus as if they were the same character. Also, you only mention blasphemy once - "Brian's followers just wanted a religion and so hailed him the "Messiah" - and that's pretty blasphemous in itself" - and in doing so give it a new definition. Life of Brian does not defame God's name. FACT.
(without the space)