Pride is much more difficult to be discerned than any other corruption because of its very nature. That is, pride is a person having too high an opinion of himself. Is it any surprise, then, that a person who has too high an opinion of himself is unaware of it?  His thinking is that he thinks that the opinion he has of himself has just grounds and therefore is not too high. If the grounds of the opinion of himself crumbled, he would cease to have such an opinion.

But, 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.

Spiritual pride takes many forms and shapes, one under another, and encompasses the heart like the layers of an onion: when you pull off one, there is another underneath. Therefore, we have need to have the greatest watch imaginable over our hearts with respect to this matter and to cry most earnestly to the great Searcher of hearts for His help.  He that trusts his own heart is a fool.

–Jonathan Edwards, “Undetected Spiritual Pride One Cause of Failure in Times of Great Revival.”