A person can commit a mortal sin…in thought, word, deed or omission…
But it requires all three things of course…grave matter…full knowledge…and deliberate consent.
Some examples of grave matter:
Murder
Choosing to Miss Sunday Mass (without excusing reason)
Adultery
Various Sins against Catholic Faith
Lust
Lying under oath in court
Serious detraction
Deny the Faith
Abortion
Contraception
Rejecting that Jesus is the Son of God
Unbelief
Euthanasia
Drunkeness (to the point of loss of reason)
etc etc
But one needs not only grave matter…one needs full knowledge and deliberate consent…
for example a person may not know that there is a grave obligation to go to Mass on Sundays and Holy Days of Obligation…they just may never have heard such.
Or they may have temptation…such are not to be confused with sins.
Or one may be truely and honestly confused…and think that one can “hold that abortion is ok” and still be living as a Catholic…due to some very real lack in their Catholic formation…and faced with some Catholics who are not living and holding what the Church holds…
It is possible that some simply are very very uninformed.
(difficult for us to understand in certain things… for us who are constantly aware of what the Church teaches…but it is possible…though I still think very difficult in the case of abortion…given the reality of it…but then again some honesty are convinced by society that such is not a child yet… )
They may not realize that there is a difference between honest “opinions” in the Church where one can have varied judgments as to say if “this war” is just …
and the teachings of the Church…which are of course not matters for opinion.
See Cardinal Ratzinger’s letter …where he talks of worthiness to present oneself for Holy Communion…and gives a couple of examples:
priestsforlife.org/magisterium/bishops/04-07ratzingerommunion.htm
(
ewtn.com/library/CURIA/cdfworthycom.htm)
It can be the case that a person may commit something that is “grave matter” but with out realizing it…not even knowing that there is any question! Like they miss Mass without realizing that such is grave and thus subjectively not commit a mortal sin.
Of course we all need to inform our consciences with the Teachings of the Catholic Church and of course to live those teachings
But some in our society may not be committing mortal sins…even though they do things that are grave matter internally or externally…due to not knowing they are grave or even knowing they should know…(we may never know if they are or are not)
(but yes there is natural moral law…so some things are more evident than others…)
Of course God is the one who will judge the person…not us. …
On our part we seek to propose Christ to them …
… and perhaps to help them come to understand why such and such is wrong…by brotherly correction …by walking along side of them…with love and understanding…and prayer etc…
and in even when things are not yet fully embraced yet…encouraging them …to seek more and more his grace…go to Mass etc (even if they can not go to Communion) to pray as Christians seeking to know Christ better and his teachings …to bring their Children up in the Faith if they are already Catholic…to read Scripture…the Catechism…to seek the answers…etc
In any case we need to propose that the great yes…which while it involves certain no’s …is well worth it

for in Christ is* true life*.
and live it ourselves…
