cant tell if you dont get the joke, or if i dont get your joke

I don't think there's much of a joke to get.

My reading is:

1) Someone (person A) said "He's not the messiah, he's..."

2) Another (person B) thought A meant that Terry Jones played the messiah (Brian) and corrected them by writing "...he's the actor that played his mum".

3) I stepped in to say, I don't think A implied Terry did was the messiah in the movie, he just referenced one of Terry's lines from the movie.

Makes sense?

Was there some joke I've missed?

Yes. The joke was that Person A's comment, replying to an article about Jones, makes it seem as if the 'he' in the line is referring to Jones, not the baby Messiah from the movie. B then subverted the expected conclusion of Jones' line "...he's a very naughty boy" with a fact about Jones himself: "he's the actor that played his mum". Person B was aware of Jones' line and played off it.

This was indeed the joke I was aiming for - though in reality it was probably as funny as coldtea thought...

I dunno, I liked it. Every joke sounds bad when someone breaks it down like I did.

I liked it too.

“Analysing comedy is like dissecting a frog. Nobody laughs and the frog dies.”

- Barry Cryer

I got the mechanics of your response (the literal answer being funny because it's true while also agreeing on the "he's not") and even found it funny (let's say 56% funny).

But I considered it premised on being an (actual, regardless of if being done as a funny quip) correction of the grandparent at the same time.

Let's analyze this further, it's fun!

Yes, you missed it, but you can probably get away with crucifixion:

https://www.imdb.com/title/tt0079470/quotes/qt0471984