Implanting false memories can work wonders on a person’s faith ability, as having many memories of successfully altering reality via will alone, boosts one’s confidence and makes it easier for them to actually alter reality, based on those false memories of success. False memories can be used to make very powerful willers who can easily overcome the faith of natural willers, just by implanting memories of having easily overcome the faith of natural willers. Likewise, false memories of failure can strip a person of their faith abilities, and depending on the nature of these memories, may enable certain abilities and not others, etc.
Memory Derogation: As the present - including memories - is effectively what determines the past in a sort of reverse causality, changing memories can change the past, if done right. One problem that arises is that the person doing the memory-changing is himself aware that he’s changing another’s memories, and so his mind will have no problem derogating the other’s memories - which, by the axiom of derogation of faith, renders those memories ineffective in affecting reality. The memory manipulator can get around this problem by having someone else in turn remove his memory of having done this - someone who isn’t aware that he had changed the memories of the first person.
Memory Conflict: Another problem is that past events generally affect other people - and false memories will run into conflict with other peoples’ true memories. And generally those with true memories will have corroborating evidence to back up their memories - or will be more numerous and consistent - and as a result the false memories would be overwhelmed, and the subject will know that his memory is a false memory, and it will cease to have any power. To avoid this potentially happening, false memories are generally designed to be as unfalsifiable as possible, such as anyone the subject defeated in their false memory is no longer alive; or that they had been meditating or practicing magic alone where none would see them; etc. The more unfalsifiable the false memories are made, the more real the memories are.