The Defense's argument basically boils down to:
Yes, CD had an affair (which was a bad decision, but not illegal.) But erratic, unstable LVD and AC (and MG and DW) twisted CD's totally normal, run-of-the-mill LDS teachings to justify the murders that THEY committed, and then they tried to pin it all on poor, naïve CD (whose first wife totally died of natural causes... LE just tried to make that look like murder later because there wasn't enough evidence to connect him to the kids' deaths).
It is to the State's advantage, then, to avoid religious discussion altogether -- it would only muddy the waters. Because while CD's beliefs may not necessarily be taught-in-Sunday-School LDS theology, they definitely have their roots in LDS history and "deep doctrines," and a lot of these ideas have been circulated by prominent LDS groups and "personalities" (see also Jodi Hildebrandt, Ruby Franke, Julie Rowe, Thom Harrison, Maurice Harker, Preparing a People, AVOW, FIRM foundation, etc) and have not been denounced by mainstream LDS leadership. If CD was only repeating what others have said and written and taught before him, then he's not really at fault for how LVD carried them to their logical conclusion, is he?
So yeah, I'd say the State is holding back. They don't want the trial to be about wacky religious beliefs. They want it to be about what CD as an individual personally did, what he personally had to gain, and what he personally knew about where the children were and what had happened to them.