A spiritual filter is an Agency character's primary form of defense, a barrier that protects their body and soul, along with every other level of existence, from harm and other unwanted effect. It's similar, in essence, to Dispel Bound in Bastard!!, and, like Dispel Bound, a powerful enough opponent can tear down someone's barriers to get their abilities through.
In Agency itself, spiritual filter are most useful against opponents on one's weight class and their power is derived from the user's willpower; a stronger will results in stronger defenses. Significantly stronger opponents can force themselves through these barriers outright, and those with potent enough abilities or extensive knowledge can circumvent them. Even with a strong will, if one has little to no magical energy at their disposal, their barriers will be easy to break.
Comprehensive Resistance List
Not every spiritual filter resists all of these by default; while they provide a baseline level of resistance to mental, spiritual, and conceptual attack, the rest must be developed over time. This typically happens as a result of being exposed to new forms of attack and evolving to deal with them, or through actively altering and building upon your barriers to provide such defenses.
Strong spiritual filters will have more powerful innate defenses and are possessed by those with very powerful wills. People with such strong wills will also be harder to affect even if their spiritual filters are torn down, as their sheer willpower still remains.