Generally, sin; specifically, impurity.

Michael Dubruiel at Annunciations writes a post entitled "Does Sin Keep Us From Seeing the Glory of God?"

Well…

"Blessed are the clean of heart, for they shall see God."

It follows that insofar as we are not clean, or pure, of heart, we won’t be able to see God.  I mean, yes, sin obscures God, but it’s specifically lack of a clean heart that does it.  The sin of impurity.

I think Christopher West uses this point to argue, specifically, that our lack of sexual purity causes us to be blind to the images of God that are other human persons, seeing them instead as objects.