Many things and ideas, like materialism, consumerism, and relativism have been pin-pointed as the greatest danger to the Church.
In my opinion, and this is merely opinion, I think the most dangerous single idea or problem for the Church is stupidity. That people refuse to learn the Faith and incorporate it into their daily lives causes the Church to weaken and even die in some areas.
Stupidity is caused mainly by two things. The first is the lack of study, (see post last week on the virtue of studiosity), and the second is sloth.
Sloth must be one of the most common evils among Catholics, both laity and clergy alike.
To not study the Faith, to ignore the overwhelming amount of information on the net and in books, CDs and DVDs cannot be explained away by a lack of time.
It is a lack of interest, or, simply, laziness.
I talk to people who watch hours of television per week, who talk to friends and text or spend time on Facebook, but never read the Scriptures or take time to pray.
This is dangerous and totally "fixable". In this last week before Christmas, examine how much time is wasted on temporal and ephemeral things, instead of on prayer and study.
Your soul may be in danger.