The pagans I've known go for the 'true for me' option, but having once been the other side of the fence I'll have a stab at explaining based on what I've seen.
I think it's a mix of church teaching and experience. For someone of a deep faith such as AB the relationship to God feels so real it's difficult to see that other people don't feel and experience the same thing. If you find the feeling of God's presence coming alive through prayer, church attendance, the sacraments etc then you can't understand why other people don't just do the same thing and get the same feeling. And the reason for wanting them to do that is complicated. On the surface you want them to have the good stuff that you do - peace, joy, support etc. But undoubtedly there is an undercurrent of wanting to be right, and wanting to please God, as AB has told us he feels 'called' to do.
I am basing some of the above on feelings I've had in the past myself. I'm not proud of having them; I'm beyond glad that I don't any longer, not least because it's actually quite a turbulent way to live. Why or where they came from I have no idea but the best way I can describe it looking back is a kind of delusion, even a bad dream. Makes no sense to me whatsoever.