Jonathan Edwards on Spiritual Pride.
Nothing sets a person so much out of the devil's reach as humility.
The first and worst cause of error that prevails in our day is spiritual pride. This is the main door by which the devil comes into the hearts of those who are zealous for the advancement of Christ. It is the chief inlet of smoke from the bottomless pit to darken the mind and mislead the judgment, and the main handle by which Satan takes hold of Christians to hinder a work of God. Until this disease is cured, medicines are applied in vain to heal all other diseases. Pride is much more difficult to discern than any other corruption because, by nature, pride is a person having too high a thought of himself.