“Grave sin” is actually not a proper term. Traditionally, according to the catechism, we have mortal/deadly sin, or grave matters. So when you say “grave sin”, it is very ambiguous as to what exactly you are referring to.
So, let me first explain what is “mortal sin” and “grave matter”. Then I will try to guess what you mean by “grave sin”.
In the last chapter of the first letter of John, he clearly stated that there are sins that are mortal, or deadly. The Church came to understand that they refer to sins that are so radically opposed to God that they sever our relationship with God and cut off the habitual gift of grace. One who has committed a mortal sin is therefore cut off from grace, and would fall into hell if unrepented. Mortal sins can be removed only by the sacrament of confession or baptism, or a perfect act of contrition.
For a sin to be mortal, the Church teaches that three conditions need to be met:
- That the act itself is of grave matter;
- That it is done knowingly, or that the ignorance of the matter is not excusable;
- That the person is free to choose it.
For #1, grave matters are:
– those action that is directly against the ten commandments: ie., idolatry, blasphemy, failing Sunday obligations, dishonouring parents, murder, adultery/fornication, theft, etc.
– actions against life: abortion, contraception, IVF, embryonic experimentation/manipulation etc.
– and those items explicitly mentioned in the catechism e.g., masturbation etc.
Notice that a grave matter itself does not automatically make a sin mortal. It requires the person to choose it knowingly and freely.
Ignorance however does not mean someone is inculpable. If someone felt that there is something wrong with this action, but refused to ask or to find out, or deliberately consulted people that would give them the answer they like or would concur with them or would give them ambiguous enough an answer in order to claim ignorance, then this person is fully culpable. However, if the person sincerely did not realize something is wrong, or that they have consulted someone in authority but that person gave them the wrong answer, then there is no culpability.
As to freedom, it can be lacking if the person is under duress, threat, or chemically addicted. Freedom is also limited for someone suffering from depression and the various forms of mental illnesses.
Whenever there is lack of knowledge or freedom, it is no longer considered a mortal sin, but venial.
In practice, however, it is often hard to tell strictly if an act is grave matter, or that the person may not be able to tell truthfully if he acted with full knowledge and consent. Some people proposed to call these serious sins. My guess is that if what you mean by “grave sin” is not mortal sin, then it is probably referring to “serious sin”.
And the wisdom of the Church is: when in doubt, go to confession. And along with that: always believe that God’s mercy is waiting for you in the confessional.