The Devil in Sheep's Clothing?

"If the devil comes to my door with his horns visible,
I will never let him in; but if he comes with his hat on
as a respectable gentleman, he is at once admitted.
The metaphor may be very strange, but it is quite true--
many a man has taken in an evil thing, because it has been
varnished and glossed over, and not apparently an evil; and he
has thought in his heart, there is not much harm in it; so he has
let in the little thing, and it has been like the breaking forth
of water- the first drop has brought after it a torrent.
The beginning has been but the beginning of a fearful end."