Such have sinned by taking pleasure in their minds but have not consented to it in deed. Here, the heart is tempted and feels pleasure in the sinfulness of the flesh, and yet opposes a rational resistance to the sinfulness so that in its innermost thought it is saddened by what pleases it, and pleased by what saddens it. 

If the soul, however, is prone to execute its evil wishes when the opportunity is offered, then such soul has sinned, not merely in thought, but also in deed. 

Clearly a sin of thought is one wherewith even given the opportunity to sin, the soul will not fall into it. 

From the example of our first parents, we see that sin is committed in three stages, namely, the suggestion of it, the pleasure experienced, and the consent. The first is the act of the enemy, the second that of the flesh, the third that of the soul. 

The devil who continuously is in wait for us suggests the evil. The flesh submits to the pleasure. Then the soul, overcome by the pleasure, consents. 

Thus, the devil suggested the act of disobedience. Eve gave in to the pleasure. Adam, overcome by the suggestion and the pleasure, gave consent. And so, it is said that the fall was accomplished by Adam.

We become aware that there is sin in the suggestion; we are overcome by pleasure and we fall by consent. 

Those who have sinned in thought must realize the gravity of such state lest feeling less horror for their state they may easily fall into greater evil. God, though, absolves sins of thought more readily because they have not reached consummation. In Scriptures, these sins are called wickedness of the heart, "Thou has forgiven the wickedness of my heart." This needs only the cleansing of the mind. 

St. Gregory the Great, Pastoral Rule, Part III, Chapter 29