Jesus is not giving an extended dissertation on soteriology. He is simply saying what the external effect looks like. And it shouldn’t be too difficult to understand the distinction between intellectual assent and approval to certain ideas, ‘believing’ them, and the essence of the heart’s desire.
The problem with many translations of Psalm 15:2 is changing the preposition to reflect modern English idiom. It should not be 'from": “…and speaks truth IN his heart.” I like the present continuous tense of the YLT (it is not just a one time thing, but a state of being):
He who is walking uprightly, And working righteousness, And speaking truth IN his heart.
We are a particularly gullible species and extremely capable of being deceived and deceiving ourselves. Recent events in the U.S. demonstrate that nicely, belief in political lies and investment in conspiracism.