Many times those who accuse others of self-deception are in fact the ones who have deceived themselves.
Case in point...
"At hearing this, the Pharisees said: “This fellow does not expel the demons except by means of Be·elʹze·bub, the ruler of the demons.” ...Jesus said to them, "But if it is by means of God’s spirit that I expel the demons, the Kingdom of God has really overtaken you...Whoever is not on my side is against me, and whoever does not gather with me scatters. For this reason I say to you, every sort of sin and blasphemy will be forgiven men, but the blasphemy against the spirit will not be forgiven."
Not surprising that the Pharisees would have deceived themselves, since their ruler is the father of self-deception.
"Either you make the tree fine and its fruit fine or make the tree rotten and its fruit rotten, for by its fruit the tree is known. Offspring of vipers, how can you speak good things when you are wicked? For out of the abundance of the heart the mouth speaks. The good man out of his good treasure sends out good things, whereas the wicked man out of his wicked treasure sends out wicked things. I tell you that men will render an account on Judgment Day for every unprofitable saying that they speak; for by your words you will be declared righteous, and by your words you will be condemned."
Matthew chapter 12
There are many "religious" people today, some leaders of groups, some not, who falsely claim to be "knowledgable" and yet have deceived themselves and others. By their fruits they are recognized.