Sermon interrupted

Excellent work locating the volume involving Sophia by name:

The next trick will be to get out of the gift shop without anyone noticing you've pinched the book! Tuck the book under your coat and calmly exit. You'll need to go somewhere private to open the book and see what clues it holds.

Entrance to the tunnels
My research shows there are tunnels beneath the basilica. The door to the tunnels is marked EMERGENCY EXIT ONLY in various languages (including English) and is located just to the right of the men's restroom across from the gift shop. Due to construction, the alarm on the door has been deactivated. Yea!

If you brought a flashlight as one of your 10 items to pack, we're in good shape. If not, you will have to use the light on your mobile phone if it has one, thereby draining power. If you did not pack either a flashlight or phone charger, return to the gift shop and purchase one or the other. [The flashlight should cost ₺18, a phone charge ₺60, and the trip to the gift shop 15 minutes.]

Once you are deep in the tunnels and don't see or hear anyone, crack open the book.
(If working remotely, click the image of the book at the top-right of this page.)