I admit, I get horribly mind-killed whenever I realize I'm reading something by CS Lewis, especially anything from The Screwtape Letters. That's because years ago, the arguments in this book were used against me by a girl I was dating as a means to end our relationship (me being non-religious), who herself was convinced by her friends and family that we should break up.

That said, I was able to read this and appreciate it more clearly if I substituted the quote like so:

Jargon, not argument, is your best ally in keeping him from the [bad guys]. Don’t waste time trying to make him think that [good guys' philosophy] is true! Make him think it is strong, or stark, or courageous— that it is the philosophy of the future. That’s the sort of thing he cares about.

If we are attempting to spread good rationality around, would it be efficient to not try to convince people that rationality was "true", but instead attempt to promote good rationality by saying that rationality is "strong, stark, or courageous -- that it is the philosophy of the future"?