Belief in the Brain: Sacred and secular ideas engage identical areas
scientificamerican.com
scientificamerican.com
Any activity related to theology could easily blend right in with this background activity. They're making the assumption that because theology is a "big issue" that it should stick out like a sore thumb on an fMRI. That's not necessary true. The average church-going Christian adult has probably heard about Jesus's life thousands of times in their life. They could easy answer a question about him without even considering the theological implications of the question. It's just another semantic detail to them.
Edit: Added second paragraph.
As you point out, it is a reasonable hypothesis that what they are measuring is simply recalling already established beliefs. The critical part is really why people come to their beliefs. Secondary to that is why people do not revisit established beliefs, and if they do, why do they maintain them.
Generally, the brain evolved to be lazy. Our ancestors needed to spend their time finding food, not questioning reality, and so evolved to make a decision and move on. We only re-examine firmly-held beliefs when a great conflict arises and even then we spend a great deal of effort rationalizing to try to resolve the conflict without overturning the beliefs.
Rational activities like science and philosophy, where you are forced to dig deeply into your assumptions, are very unnatural. That's why they're so hard to learn to do right.
... providing further proof that evaluating truth or falsity is independent of the content of the statement in question.
The key distinction I'm going after here is whether a fact believed on authority or believed because of empirical proof. I believe that the earth rotates around the Sun and that gravity accelerates things at 9.8m/s^2 and that there once existed a fellow named Kublai Khan (who in Xanadu did a stately pleasure dome decree). But I believe all of these things because I read about them in books written by experts, not because I made observations with a telescope and did the orbital calculations, or because I've timed dropping objects from various heights, or because I've examined the historical record of the Yuan Dynasty.
Presumably an expert who has done the empirical work will not accept these facts on the basis of belief, but on the basis of real-world observed evidence/proof. That ought to trigger a different part of the brain than in someone who believes a scientific fact based on the authority of scientists and the scientific method.
To the brain, beliefs are beliefs, whether they concern god, angels or motor oil.
Not all cognition about religious things involves a "mystical experience". In fact, theologians tend to be an incredibly dry bunch and study their creeds with all the emotional investment of an accountant.