What are mortal sins?

I want to know what the mortal sins are because I’m not sure I’m just starting to become serious about my faith even considering becoming a nun

‘The mortal sins’ cannot be listed. In order for a sin to be mortal it must be (1) of grave or serious matter, (2) and committed with full knowledge (that it is mortal) and (3) deliberate consent.

Many things are sins but the degree of seriousness of each one depends on circumstances. Stealing can be a mortal sin - but stealing a pencil from work and embezzling $10,000.00 are different degrees of the sin of stealing - one may be venial whereas the other mortal.

Full knowledge means knowing that what is being done is of serious or grave matter - before and/or at the time the action takes place. Realizing only later that what was done was of grave matter mitigates this full-knowledge requirement.

Deliberate consent means that one has not been coerced into committing the act. It also means that the person committing the act is of reasonable mind. Habits and addictions may mitigate this requirement as well.

You may wish to consult the Catechism of the Catholic Church for a little more information, paragraphs 1849-1864. You can find the CCC online if you don’t have a copy handy.


Catechism of the Catholic Church 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.”

So, one commits a mortal sin by acting in “grave violation of God’s law” (CCC 1855), possessing knowledge of the sinfulness of the act and deliberately choosing to commit it.

When you say…

I assume you’re asking about grave matters-- which will be mortal sins when the other conditions stated above are met. So… as for actions in terms of their object (i.e. “grave matter”) that are gravely sinful? A few examples, in no particular order:

Apostasy, heresy, blasphemy, false oaths, unjust killing (e.g. murder, abortion, euthanasia, suicide),
homosexual acts, any other acts involving deliberate sexual pleasure outside/before marriage (e.g. fornication, masturbation, lust), contraception, theft (venial if the matter is light), serious lying, wishing grave harm on another, serious dishonour of one’s parents, failure to fulfill one’s Sunday obligation, divination, witchcraft or occultism, and so on.

The Catechism contains articles on each of the Ten Commandments which outline some of the offences against them (see Part 3: Life in Christ, Section 2: The Ten Commandments). It will probably be a good place to start as you continue to learn more about various aspects of the faith.

If you have specific questions about sin or theological matters, it may be a good idea to talk to your priest.

God bless.

[Edited for formatting]

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From the Catechism: (parenthesis added :wink:)
"For a sin to be mortal three things are necessary,

  1. A grievous matter
    2.Sufficient reflection (you know this sin to be grievous, and you know you shouldn’t do it)
  2. Full consent of the will." (you’re gonna do this bad thing anyway no matter what)

Here’s what the latest Catechism of the Catholic Church states

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

Note, according to a my last confessor, who was also a Dr of Theology, he told me, “this is not infallible, but a guide in discerning if a sin is mortal or not.”


Yeah, I think that reading the conditions there too strictly can be misleading; in general, I would say if a person thinks they may have committed a mortal sin, they should refrain from receiving the Eucharist until confession can be made. Better than approaching the sacrament unworthily and committing another mortal sin by doing so.

Mortal sins are sins that kill us spiritually.

They are objectively grave matter. In addition to being objectively disordered (which is enough to make them objective mortal sins in some terminology), for them to be subjectively mortal (i.e., kill you spiritually), they have to be done with sufficient knowledge of the sinfulness and deliberate will to do it.

Think about the 10 commandments, and perhaps speak to a priest.

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