Humility is not thinking less of yourself, it's thinking of yourself less.

Although Rick Warren's quote in 2002 is the one most often misattributed to Lewis, two very similar quotes predate Warren's: (1) "People with humility don’t think less of themselves, they just think of themselves less” (Ken Blanchard, Mission Possible, 1999) and (2) “Humility is thinking less about yourself, not thinking less of yourself” (Peter Kreeft, Back to Virtue, 1992).