Three conditions are necessary for mortal sin to exist:
Grave Matter: The act itself is intrinsically evil and immoral. For example, murder, rape, incest, perjury, adultery, and so on are grave matter.
Full Knowledge: The person must know that what they’re doing or planning to do is evil and immoral. For example, someone steals a postage stamp, thinking that it’s only worth 50 cents. She knows that it’s sinful, but if she’s unaware that the stamp is rare and actually worth $1,000, she’s not guilty of mortal sin but of venial sin.
Deliberate Consent: The person must freely choose to commit the act or plan to do it. Someone forced against her will doesn’t commit a mortal sin. For example, a woman told she’s giving a minor shock to another person when in fact she is administering tortuous electrical jolts is not guilty of a mortal sin (although she may feel guilty if she finds out the truth).
A mortal sin is the complete turning away from God and embracing something else in place. It’s deadly to the life of grace, because it insults the honor of God and injures the soul of the sinner. Mortal sin is like a malignant tumor or a critical injury that’s lethal to the spiritual life.Venial sins are any sins that meet one or two of the conditions needed for a mortal sin but do not fulfill all three at the same time, or they’re minor violations of the moral law, such as giving an obscene gesture to another driver while in traffic.
Venial sin only weakens the soul with sickness but doesn’t kill the grace within. Venial sins aren’t deadly to the life of grace but, like minor infections in the body, if casually ignored and left untended, may deteriorate into a more serious condition. For example, someone who tells so-called white lies commits venial sin, but if he does it long enough, it’s much easier for him to be tempted to tell a big lie later on that would in fact be a mortal sin, such as cheating on a test or on his income tax return.