Augustine of Hippo (354-430) regarded sin as "...a word, deed or desire contrary to the eternal law." Catholic moral theology divides sin into two parts, primarily on the basis of degree and effect:

Mortal ('Deathly') sin - This type of sin is the most serious as it involves loss of sanctifying grace. A person who dies with unremitted mortal sin would be in danger of eternal separation from God in Hell. Therefore, the church requests those who commit such sins to undertake the Sacrament of Reconciliation, in order to restore themselves to a 'state of grace'.

"For a sin to be mortal, three conditions must together be met: Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."

"Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honour your father and your mother. The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger."

Venial ('Pardonable') sin - Positively, this sin does not directly destroy the relationship with God. Rather, it weakens that relationship. Unremitted venial sins can affect the duration spent in Purgatory. Therefore the church encourages confession of these types of sins as well.

"Without being strictly necessary, confession of everyday faults (venial sins) is nevertheless strongly recommended by the Church..."

"One commits venial sin when, in a less serious matter, he does not observe the standard prescribed by the moral law, or when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent."