Truth and Sin in the Heart

Psalm 4:

What you say in your hearts, regret upon your beds...

Psalm 14:

O Lord, who shall dwell in Your tent? Or who shall rest upon Your holy mountain?
He who walks without blemish, and does what is right;
Who speaks the truth in his heart, and deceives not with his tongue...

Psalm 15:

I will bless the Lord who has given me understanding; even in the night my heart exhorts me...

Matthew 5:28:

But I say to you, that whosoever shall look on a woman to lust after her, hath already committed adultery with her in his heart.

Matthew 15:18-19:

But the things which proceed out of the mouth, come forth from the heart, and those things defile a man. For from the heart come forth evil thoughts, murders, adulteries, fornications, thefts, false testimonies, blasphemies.

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St. John Chrysostom, commenting on Matthew 5:28, says:

If you permit yourself to gaze often on fair countenances you will assuredly be taken, even though you may be able to command your mind twice or thrice. For you are not exalted above nature and the strength of humanity. She too who dresses and adorns herself for the purpose of attracting men's eyes to her, though her endeavor should fail, yet shall she be punished hereafter; seeing she mixed the poison and offered the cup, though none was found who would drink thereof. For what the Lord seems to speak only to the man, is of equal application to the woman; inasmuch as when He speaks to the head, the warning is meant for the whole body.

St. Jerome, commenting on Matthew 15:18, writes:

[...] by this passage we may refute [p. 560] those who think that evil thoughts are suggestions of the Devil, and do not spring from our proper will. The Devil may encourage and abet evil thoughts, but not originate them. And if he be able, being always on the watch, to blow into flame any small spark of thought in us, we should not thence conclude that he searches the hidden places of the heart, but that from our manner and motions he judges of what is passing within us. For instance, if he see us direct frequent looks towards a fair woman, he understands that our heart is wounded through the eye.

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These passages should lead us to reflect on how easy it is for us to corrupt our hearts, to harden them with sin. We should be inspired to a greater sensitivity of our inner motions. The mouth speaks what is first in the heart. We don't even need to outwardly speak evil things to our neighbors. Letting our heart speak these evils is enough to sin and corrupt ourselves and defile love. Therefore, the Holy Ghost tells us to regret the evil that we speak in our hearts. Those shall dwell with the Lord who speak truth in the heart and, with their hearts full of truth, proceed to speak the truth outwardly to their neighbors by not deceiving them.

Moral evil originates from the heart. The responsibility falls on us. The devil may suggest evil or fan it into flame as St. Jerome tells us, but we decide what goes into our hearts and what comes out of them.