Wenn man einmal das Böse bei sich aufgenommen hat, verlangt es nicht mehr, daß man ihm glaube.
When one has once accepted and absorbed Evil, it no longer demands to be believed. [Kaiser/Wilkins]
Once we have taken Evil into ourselves, it no longer insists that we believe in it. [Hofmann]
The first verb is the tricky one, since it means to receive, specifically to receive persons (as opposed to acts). Receiving someone, and here evil is clearly personified, and taking someone or something into oneself are not quite the same. If I take evil into myself, then I become evil, don't I? Whereas what is under discussion here seems to be knowing and accepting evil, rather than doing or being evil.
It is a characteristic of evil that people do it while claiming not to be doing it. When you are unaware of it, it demands to be received. Once it is received, it hides.
This aphorism is also telling us that evil does demand we believe in it, so long as we do not receive it. Evil does not allow itself to be passively ignored, and, if it is actively ignored, that means it has been "received."
If this evil is the same as the imposed negative of the previous aphorism, then receiving it would be the flipside of having it imposed on you. This may mean that one cannot be subject to this imposition without first allowing the negative.