I wasn't able to find much lore regarding effects of holy symbols apart from "allows divine spellcasters to channel deity energy" and "require blessing from associated church to work". The logic seems to be that the symbol acts as a target (i.e. casting "focus") to direct the divine magic being channeled, and must therefore be paired with the deity like a bluetooth device to function.
What would happen if an underground church of an evil god went around carving the god's emblem into the forehead of everyone they could find, blessing it each time?
There is precedent that gods have agency of their own and can perform varying levels of action upon mortals without needing assistance from divine spellcasters, so would this allow the god to channel divine magic into/through these people? Would it simply make them easier to target?
What would be the difference if the symbol had instead been applied to a clumsy peasant tripping forehead-first onto a badly (or well) placed red-hot branding iron for the symbol? In this case what if the inadvertently branded symbol was later secretly blessed by the relevant church?
In the case of a birthmark matching the holy symbol, it was not created specifically to be the symbol but matches coincidentally. Does this differ from an application of the actual symbol, accidental or intentional?
To be very specific, I'm asking about any implications that would differentiate well known holy symbols widely associated with a particular deity from any other symbol (or quasi-religious icon) which could be constructed in an identical manner but lacks any preexisting association with a god. This is not about mundane or social effects like identifying membership in a certain group.