"He that is not against us is for us." This saying is the paradoxical complement to his first saying. For it makes the common sense point that, on the one hand, there is no salvation outside the Church, yet, on the other hand, we puny mortals do not know where "outside" is.
I thought you both might like this essay after all the back and forth on the Lutheran Minister thread (which I gave up on). A little different take on the same sort of issue.