The heart is deceitful above all things, and desperately wicked: who can know it? (Jer. 17:9)
At the beginning of his dialogue De tranquillitate animi (“On Tranquility of Mind”), Seneca the Younger 1 has his interlocutor Serenus, in discussing his weakness of will in doing what is good and right, comment on the dangers we face through flattering ourselves about our own achievements. As often, Seneca, who was (spuriously) claimed in antiquity to have corresponded with the Apostle Paul, is an astute observer of human nature.
I imagine many people could have achieved wisdom if they had not imaged they had already achieved it, if they had not dissembled about some of their own characteristics and turned a blind eye to others. For you have no reason to suppose that we come to grief more through the flattery of others than through our own. Who has dared to tell himself the truth? Who even when surrounded by crowds of toadying sycophants is not his own greatest flatterer? (De tranquillitate animi 1, trans. C.D.N. Costa)