This should go without saying, but out of caution, I will mention this here explicitly:
Gustav's hate against Catholics is not shared by the author of this comic!
Yep, you got it! Gustav is mentally stuck in the era in which he died, namely the 17th century. (The next strip will reveal a few more details about that.)
(I also have nothing against Catholics, for what it's worth.)
It's nice to see that particular detail about Mira.