I would say we are

We were not talking about Furby's religious beliefs here. In fact, I have never heard Furby say a thing about Mary being 12. It is you and others who introduced this idea, not Furby. I don't discuss anything with Furby because frankly I see little point in talking to him. We clearly have almost nothing in common. So yeah, this has absolutely nothing to do with Furby. Nice try.

Furby has many times stated that he is Roman Catholic. Seamus introduced Mary's age because he is RC. If we had known that he was a non-Christian, he wouldn't have mentioned Mary. Your caveat that the existence of Jesus and Mary is not proven, while true for history, is not relevant in the context of Furby who as a RC has to accept them as historical people. In this context, there is no debate. It is canon. Same as you just cannot say that Luke was not Anakin's son - that is canon too.