Sound Reasoning From a Surprising Source
The greatest dangers are always so particularly dangerous because they go unnoticed. It is the same in the Church. She is always threatened by dangers. The greatest are those from within; all those from outside only become dangerous to her if they touch upon a weakness within. The promise that the gates of hell will never prevail against the Church does not promise her constant visible, "empirical" strength and immunity, but promises the power which is God's alone in the weakness and vulnerability of the men who are her members. When men feel safe and assured because "nothing can ever really happen to the Church," then they always find out sooner or later that indeed nothing can "happen" to the Church, who is in God's hand, but quite a lot can happen to the men who out of idleness or timidity do nothing and rely on this.
--- Karl Rahner, S.J. , Nature and Grace: Dilemmas in the Modern Church (NY: Sheed and Ward, 1964), p. 5.