I have a question about mortal sin. I know that for a sin to be a mortal sin, it must be committed with full knowledge and deliberate consent. Now, suppose I engaged in premarital sex and didn’t know it was a sin. Then, the “full knowledge” requirement is not met and I have not committed a mortal sin. Then, I later find out that premarital sex is a mortal sin. Even though this sin was committed without full knowledge, do I still need to confess it? Or is this only if I commit the sin after knowing it is a mortal sin?
You will have to ponder it. Note in the text following that “no one is deemed to be ignorant of the principles of the moral law,”
Look at these items from the Catechism of the Catholic Church.
1860 Unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest.
I. THE NATURAL MORAL LAW
1956 The natural law, present in the heart of each man and established by reason, is universal in its precepts and its authority extends to all men. It expresses the dignity of the person and determines the basis for his fundamental rights and duties:
For there is a true law: right reason. It is in conformity with nature, is diffused among all men, and is immutable and eternal; its orders summon to duty; its prohibitions turn away from offense . . . . To replace it with a contrary law is a sacrilege; failure to apply even one of its provisions is forbidden; no one can abrogate it entirely.9
1960 The precepts of natural law are not perceived by everyone clearly and immediately. In the present situation sinful man needs grace and revelation so moral and religious truths may be known "by everyone with facility, with firm certainty and with no admixture of error."12 The natural law provides revealed law and grace with a foundation prepared by God and in accordance with the work of the Spirit.
More On Mortal Sin
1857 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.” 131
1858 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, Honor your father and your mother.” 132 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.
1859 Mortal sin requires
full knowledge and
complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God’s law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart 133 do not diminish, but rather increase, the voluntary character of a sin.