須防仁不仁 Guard Against the Unkind
行者道：「怕他怎的？」三藏道：「說那裡話？『不信直中直，須防仁不仁。』我也與你走過好幾處松林，不似這林深遠？行者道：「怕他怎的？」三藏道：「说那里话？『不信直中直，须防仁不仁。』我也与你走过好几处松林，不似这林深远？
逐句解 Line by line
Pilgrim said, "What is there to fear in it?"
Five characters, with no argument in them at all — only contempt for the question.
Tripitaka said, "What kind of talk is that?
說那裡話 is a fixed protest, and its 那 is the question word nǎ — literally "where does that talk come from?" Chinese still says it today when somebody has been too casual about something serious.
'Do not trust the straightest of the straight; be on your guard against the kind who are not kind.'
A rhyming warning built on doubling each word: 直中直, straight within straight, against 仁不仁, kind and not-kind. The point is that appearances of virtue are exactly what a fraud will present, so the more upright something looks the more carefully you should check.
You and I have walked through a good many pine woods together — was any of them as deep and far-reaching as this one?
He reasons from experience, which is Pilgrim's own method turned back on him. Tripitaka has walked through pine woods before, and this one is not like those.
This proverb is the chapter in miniature, and the novel knows it. Do not trust what looks upright; guard against apparent kindness. Tripitaka quotes it here to justify being afraid of a wood.
Within a few pages he will be shown something that looks exactly like innocence in distress, and he will not apply his own proverb to it for a moment. Pilgrim will apply it for him, and be shouted down for it. The novel very rarely spells out its morals this far in advance, and when it does it is usually about to show you somebody failing to follow one.