Undiscerned Spiritual Pride
Those who are most zealous in the cause of God are the most likely to be targeted as being filled with pride. When any person appears, in any respect, to be noticeably excelling others in his Christian walk, odds are ten to one that it will immediately awaken the jealousy of those about him.
They will suspect (whether they have good reason or not) that he is very proud of his goodness and that he probably thinks no one as good as he is, so that everything he says and does is observed with this prejudice.
Those who are themselves cold and dead, and especially those who have never had any experience of the power of godliness on their own hearts, will easily entertain such thoughts of the best Christians. This arises from nothing less than a secret hostility against essential and fervent holiness.
But the zealous Christian should take heed that this does not prove a snare to him, and the devil take advantage of it to blind his eyes from beholding the true nature of his heart and to think that because he is charged with pride wrongfully and with an unkind spirit, that such charges are not sometimes valid…
…Because of the nature of spiritual pride, it is the most secret of all sins. There is no other matter in which the heart is more deceitful and unsearchable and there is no other sin in the world that men are so confident in. The very nature of it is to work self-confidence and drive away any suspicion of any evil of that kind. There is no sin so much like the devil as this for secrecy and subtlety, and appearing in great many shapes that are undetected and unsuspected.
-Jonathan Edwards : http://www.ccel.org/ccel/edwards/works1.ix.v.i.html