What is Mortal Sin?

Q - What exactly qualifies as a mortal sin? According to a website I read, breaking any of the 10 Commandments along with a slew of other things is a grave sin? In this light, what if we've taken the Eucharist in mortal sin before without the prior knowledge that we weren't supposed to?

A - Thanks for the question.

For a sin to be mortal, it must meet the following criteria:

1. The act must be grave (serious)
2. Must be committed with full knowledge
3. The sin must be committed with deliberate consent

If these three conditions are met, then the sin is mortal - the life of grace is "killed" in our souls. This is a choice, on our part, to turn our backs on God.

If these conditions are not met, then the sin is venial - it "wounds" us.

While all sins are bad - mortal sins are deadly to us. Thus, the apostle John says:

"If anyone sees his brother sinning, if the sin is not deadly, he should pray to God and he will give him life. This is only for those whose sin is not deadly. There is such a thing as deadly sin, about which I do not say that you should pray. All wrongdoing is sin, but there is sin that is not deadly."
- 1 John 5:16-17

Notice the distinction between those sins that kill us spiritually and those that do not.

St. Paul has several lists of sins that will keep us from heaven and are considered grave sins.

"Now the works of the flesh are obvious: immorality, impurity, licentiousness, idolatry, sorcery, hatreds, rivalry, jealousy, outbursts of fury, acts of selfishness, dissensions, factions, occasions of envy, drinking bouts, orgies, and the like. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God."
- Galatians 5: 19-21

More:

"Do you not know that the unjust will not inherit the kingdom of God? Do not be deceived; neither fornicators nor idolaters nor adulterers nor boy prostitutes nor sodomites nor thieves nor the greedy nor drunkards nor slanderers nor robbers will inherit the kingdom of God."
- 1 Cor 6: 9-10

So, we can see that there are quite a few acts that are gravely immoral and are thus mortally sinful.
Thus, breaking any of the Ten Commandments may be a mortal sin. But, we must understand that each of the three conditions must be met.

The Catechism says this about grave matter:

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."

Now, if you were to receive Communion in the state of mortal sin, then it is very serious thing. But, if you did so out of ignorance, your culpability is lessened. The Catechism comments on this as well.

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."

When in doubt about mortal sin - go to confession.
Also, when in doubt - do not receive the Eucharist.
I hope this helps.

Related Post:
**Is Confession To a Catholic Priest Necessary or Can You Go Straight To God?