Question:
Answer:
In Matthew 5:22, 27-28, Jesus teaches:
But I say to you that every one who is angry with his brother shall be liable to judgment; whoever insults his brother shall be liable to the council, and whoever says, “You fool!” shall be liable to the hell of fire (5:22).
You have heard that it was said, “You shall not commit adultery.”But I say to you that every one who looks at a woman lustfully has already committed adultery with her in his heart (5:27-28).
It’s possible for a sinful thought to be a mortal sin, such as if a person in anger plots out at length the murder of another person. But recall that mortal sin involves not simply grave matter but full knowledge and complete consent (CCC 1857-61). Because anger is often a sin of passion, a person may quickly become angry and then their anger may fairly quickly subside. This would often indicate that their anger lacked full knowledge and complete consent and thus wasn’t a mortal sin. (In addition, it’s possible to have righteous anger and not sin at all [Eph. 4:26-27]).
Similarly, a lustful thought could be a mortal sin, but given that these thoughts often pass fairly quickly could likely mean that a mortal sin hasn’t taken place.