An open mind is a wonderful character trait. Like any good thing, however, a person can take it too far. As C.S. Lewis says int he quote above, an open mind about foundational issues is idiocy.
Recently, the Presbyterian Church (PC USA) voted to amend its policy of requiring clergy to be faithful to the Biblical mandate of sex only within marriage between a man and woman. In other words, each regional presbyteries can now decide upon the ordination of gay or outside-of-marriage sexually active clergy. Here is an article talking about it: http://www.cbn.com/cbnnews/us/2011/May/Presbyterian-Church-Clears-Way-for-Gay-Clergy-/.
C.S. Lewis hits the nail on the head. Tolerance and an open mind are generally wonderful traits, but they should never extend to foundational issues. When it comes to foundational issues, we should be as stubborn as mules. The Presbyterian Church folded on a foundational Biblical mandate.
I have to point out one thing, however. I have gay friends and I am compelled to say that the church today does a horrible job of dealing with gay people who are genuinely searching and struggling. Nevertheless, the solution is not to begin to allow gay people in leadership roles.