56 Q. How many things are necessary to make a sin mortal?  A. To make a sin mortal three things are necessary:  a grievous matter, sufficient reflection, and full consent of the will.

“Grievous matter.”  To steal is a sin.  Now, if you steal only a pin the act of stealing in that case could not be a mortal sin, because the “matter,” namely, the stealing of an ordinary pin, is not grievous.  But suppose it was a diamond pin of great value, then it would surely be “grievous matter.”  “Sufficient reflection,” that is, you must know what you are doing at the time you do it.  For example, suppose while you stole the diamond pin you thought you were stealing a pin with a small piece of glass, of little value, you would not have sufficient reflection and would not commit a mortal sin till you found out that what you had stolen was a valuable diamond; if you continued to keep it after learning your mistake, you would surely commit a mortal sin.  “Full consent.”  Suppose you were shooting at a target and accidentally killed a man:  you would not have the sin of murder, because you did not will or wish to kill a man.

Therefore three things are necessary that your act may be a mortal sin:  (1) The act you do must be bad, and sufficiently important; (2) You must reflect that you are doing it, and know that it is wrong; (3) You must do it freely, deliberately, and willfully.

57 Q. What is venial sin?  A. Venial sin is a slight offense against the law of God in matters of less importance, or in matters of great importance it is an offense committed without sufficient reflection or full consent of the will.

“Slight,” that is, a small offense or fault; called “venial,” not because it is not a sin, but because God pardons it more willingly or easily than He does a mortal sin.  “Less importance,” like stealing an ordinary, common pin.  “Great importance,” like stealing a diamond pin.  Without “reflection” or “consent,” when you did not know it was a diamond and did not intend to steal a diamond.

58 Q. Which are the effects of venial sin?  A. The effects of venial sin are the lessening of the love of God in our heart, the making us less worthy of His help, and the weakening of the power to resist mortal sin.

“Lessening of the love,” because it lessens grace, and grace increases the love of God in us.  It displeases God, and though we do not offend Him very greatly, we still offend Him.  “Weakening of the power to resist.”  If a man is wounded, it will be easier to kill him than if he is in perfect health.  So mortal sin will more easily kill a soul already weakened by the wounds of venial sin.

59 Q. Which are the chief sources of sin?  A. The chief sources of sin are seven:  Pride, Covetousness, Lust, Anger, Gluttony, Envy, and Sloth; and they are commonly called capital sins.