The Truth is that all of reality is a Lie. The Truth exists even if it is not perceived and accurately states nothing is real, everything is possible and the horrifying consequences of those both being true. Even the various supernatural realms are extremely limited in their expression of the truly endless, formless and meaningless infinite possibilities that exist. In particular relevancy for the Mage venue, the Supernal Realm is part of a larger Lie, that there is any reality at all. This and those realities are no more real that the possibility of infinite others and they all exist in a sense, yet that sense is utterly meaningless given the infinite sea of others they find themselves adrift in.
The Truth isn’t malicious but tapping into it is tapping into that which is between, around and throughout every possibility including realities anathema to our own reality and by its nature destroys the definition and distinction that holds any world and a sane mind together.
Once the Truth is known (represented by the Reality Blasphemy merit or the Cultist of Things-That-Must-Not-Be merit) it infects at person’s soul and it has never been successfully removed.
It births Outer Things are based on these aspects of the Truth; Nothing is real and everything is possible.
Cults and the Spawn of the Truth are violently opposed by the seers of the Throne because the Truth unravels the Lie and weakens the rule of the Exarchs. Do not take this to mean that the Truth is beneficial, it is horrific inhuman force of pain and destruction that if it is ever ultimately fully realized will reduce whatever contains it into a meaningless nightmare beyond imagining.