A. The separation between good and evil
An evil act is a sin; it is always morally wrong to choose to commit any sin. A good act is a permissible act; it is not morally wrong to choose to commit any good act. The word “evil” is used in Catholic ethics to refer to any morally bad act, even if it is only a small sin.
In Catholic ethics, an act is a knowing choice. Every knowing choice is either good or evil, moral or immoral, permissible without sin or sinful. Sin is a knowingly chosen bad act.
God is good and unchanging. All good acts are good because they are in agreement with the love of God above all else, and the love of your neighbor as yourself. All evil acts are evil because they are in conflict with the love of God, and the love of neighbor as self.
A good act cannot change into a bad act, nor can a bad act change into a good act: good is good, and evil is evil. A morally evil (bad) act cannot be justified by another act, regardless of whether that other act is committed before, during, or after the bad act.
A good act cannot justify a bad act, nor can a bad act corrupt a good act. If a person commits a thousand good acts and one bad act, the good acts remain good, and the bad act remains bad. If a person commits a thousand bad acts and one good act, the bad acts remain bad, and the good act remains good.
B. Three sources of morality
There are only three things that can possibly make any act, any knowing choice, a sin: (1) your intention, (2) the type of act that you are choosing, and (3) the consequences that you reasonably anticipate will occur.
(1) Your intentions:
If any act is chosen with a bad intention, the act is a sin. It is always sinful to act with a selfish, harmful, malicious, or otherwise bad intention. To be moral, any act must first have only good intentions. If your intention is bad, you should change your intention.
(2) The type of act that you are choosing:
When you knowingly (deliberately, voluntarily, intentionally) choose a bad type of act, you choose to sin. Certain types of acts are morally wrong, regardless of intention or circumstances. Some immoral types of acts are only a small sin, such as a white lie. Other immoral types of acts are a grave sin, such as murder. When the type of act is bad, the knowing choice of that act is always a sin. If the type of act is bad, you should choose a different type of act.
(3) The circumstances of your act, especially the consequences
When persons choose an act, they reasonably anticipate that certain consequences will result. But if the reasonably anticipated bad consequences (harm) morally outweigh the reasonably anticipated good consequences (benefits), then the choice of that act is a sin. It is always a sin to choose to act, when you realize that your choice will do more harm than good. If the act will do more harm than good, then do not act, or choose a similar good act with different circumstances.
To be moral, all three sources of morality must be good. If any one source is bad, the act is always a sin.
C. Sincere mistakes
God is unchanging Truth and Goodness. And so, morality is not entirely subjective. There is an objective reality to good and evil. Some acts are objectively sinful; it is truth before God that these bad acts are incompatible with the true love of God, neighbor, self. Other acts are objectively moral; it is truth before God that these good acts are compatible with the true love of God, neighbor, self.
However, for a human person to be guilty of committing an objective sin, the person must have some degree of knowledge or sincere belief that the act is immoral, and must choose that act with some degree of free will. If an objective sin is chosen without any exercise of free will, or without any knowledge that the act is a sin, then the person is not guilty of actual sin. The act remains objectively immoral, but the person has no culpability. Only actual sin includes culpability (guilt).
If an objective grave sin is chosen with an exercise of free will that is less than full, or with a knowledge of the immorality of the act that is less than full, then the act is an actual sin, but the culpability is venial (light), not mortal (grave). In this case, the sin is an objective mortal sin, but the choice of that sin is an actual venial sin. Only actual mortal sin is sufficiently grave to deserve eternal punishment in Hell, if the person does not repent.
However, a person cannot justify committing one grave sin after another, by the mere claim that his conscience is clear. It is not unusual for human persons to be in bad conscience. Often this occurs because they have used various rationalizations and excuses to justify their own knowingly chosen immoral acts. Some persons are dishonest with themselves.
For a Catholic Christian to be in good conscience, he or she must seek and obtain knowledge of morality from the teachings of the Catholic Church. He must form his conscience in the light of the teachings of Jesus Christ, found in tradition, the Bible, and official Church teachings. Those Catholics who knowingly reject what the Church definitely teaches on morality are committing an objective sin. God will judge their consciences and their souls.