Pride

The church fathers speak of pride as the most basic sin, the one that leads to so many others.  You can see this especially in religion.  So often we think we have such great understanding, knowledge, and wisdom when it comes to the things of God.  We see things that happen and we want to interpret them.  Unfortunately, we interpret them according to our own extremely limited perspective.  We can actually see and understand so little.  We have lived such a little tiny slice of reality: small in time, small in location, small in influence, small in everything.  Yet we think that we can look at the good and bad things that happen and rightly interpret them.
For the Catholic and Protestant, this is especially tragic, because they ultimately "must" look to their own interpretation.  For the Orthodox, we can look to and trust in the body of Christ.  Jesus is our head and the church is his body.  He has provided for, protected, and given the Holy Spirit to lead, the church.  We don't have to depend upon on own ridiculously small understanding to try to make sense of the huge picture of history.  God has given us the church as the pillar and foundation of truth.