“The words of his mouth are trouble and deceit; he has ceased to act wisely and do good.” -Psalm 36:3
We are first introduced to the transgressor by way of his heart (vs. 1), and then by way of his intellect (vs. 2). Finally, in verse 3, we are introduced to his will ,and subsequently, his actions. He causes trouble with his mouth by speaking deceitfully (i.e., gossip, slander, lies). Because he is a deceiver at the heart and mind levels, he cannot act wisely or do good.
What a man believes in his heart, then justifies with his reason will ultimately be revealed in his words and deeds. This is why Jesus said,
“The good person out of the good treasure of his heart produces good, and the evil person out of his evil treasure produces evil, for out of the abundance of the heart his mouth speaks.” -Luke 6:45
And, “Beware of false prophets, who come to you in sheep’s clothing but inwardly are ravenous wolves. You will recognize them by their fruits. Are grapes gathered from thornbushes, or figs from thistles? So, every healthy tree bears good fruit, but the diseased tree bears bad fruit. A healthy tree cannot bear bad fruit, nor can a diseased tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Thus you will recognize them by their fruits.” -Matthew 7:15–20
This same pattern works toward faith and godliness as well. As Anslem noted in his Proslogion, “Fides quaerens intellectum,” which means “faith seeking understanding.” Anselm argued, in what today is called the Ontological argument for God, that Christians believe by faith but can then reason their way to make sense of what they believe. That is, that-than-which-there-is-nothing-greater-than we call God. And God must exist in reality because if that-than-which-there-is-nothing-greater-than exists in our mind only, it would be less than that-than-which-there-is-nothing-greater-than that exist in reality.
Belief in the heart leads to reason in the mind which leads to action in both word and deed.