I apologize for the mix-up earlier. In the scene you mentioned, after Mandy says "He's not the Messiah. He's a very naughty boy! Now go away!", Brian responds:
"I think I am the Messiah, Mum."
Then, Mandy says "He is! He is the Messiah!", to which the crowd responds in unison "Ah, he is! He is the Messiah!".
I hope this is the correct dialogue you were looking for!