ToT

My genes are superior.

Consider yourself blown the fr*ck out.

Order of the Sword is a cult hidden away on Fortuna but it does imply the existence of some organizations that specialize in hunting demons.
The demon world is not omnipresent, while demons are able to regularly cross over into the human world they are infrequent enough that a handful of demon hunters in a specific region can handle most demon-related incidents (look to the anime for an example where Dante does a lot of traveling to get to different cases). The anime establishes that powerful demons have a hard time getting to the human world so they would rely on lesser demons to open a door for them. Most incidents involving demons would be either unintelligent lesser demons that are more direct threats but not that powerful or intelligent lesser demons that are plotting to open a gate, both of which you could plausibly deal with using humans.
A devil hunter organization like the Order would likely need to send like 3-5 hunters on average for a single lesser demon, though.
Keeping a masquerade would probably be possible through most of the events we directly see, up until the Qliphoth appears in Redgrave. It'd probably be something that a demon would even want, since it makes it easier to operate if most of humanity doesn't believe they exist.

>>533866772
This is my wife.

Also my epic 3x3 btfos your entire timeline