騰雲駕霧 A Fiend Without Cunning Has No Power
眾僧道：「老爺，妖精不精者不靈。一定會騰雲駕霧，一定會出幽入冥。古人道得好：『莫信直中直，須防人不仁。』众僧道：「老爷，妖精不精者不灵。一定会腾云驾雾，一定会出幽入冥。古人道得好：『莫信直中直，须防人不仁。』
逐句解 Line by line
The monks said, "Your reverence, a fiend that is not cunning is not potent.
The line turns on a single character used twice: 精 is the fiend itself and also the cleverness that makes it one. Their point is that anything strong enough to be a spirit is by definition too clever to be caught easily.
It will certainly know how to mount the clouds and ride the mist; it will certainly know how to pass into the dark and enter the shades.
騰雲駕霧 is the standard four-character phrase for flight by magic, and it is still current Chinese for having one's head in the clouds. 出幽入冥 pairs it with movement between the living world and the dead.
The ancients put it well: 'Do not trust the straightness of what looks straight; be on guard against the man who is not humane.'"
得 is the unstressed de joining the verb to its assessment — said, and said well. 中 is plain zhōng, within. The proverb is a warning that appearances are worked at, which in a chapter with a disguised fiend asleep behind the guardian kings is aimed straighter than the speaker knows.
The monks are being careful with a guest who has just called them cowards, and their argument is shrewd: you cannot fight this thing by being strong, because anything worth calling a spirit is already cleverer than that.
The proverb they choose is the sharpest thing in the passage. Do not trust what looks straight. Every reader knows that a young woman was cut down from a tree three days ago because she looked like a victim, and that she is asleep in this building.
The monks quote it as general worldly caution. It is in fact a precise description of what has been done to their monastery, and none of them can see it.