Galatians 5:18-25 / Luke 11:42-46
It may not be that necessary to define what a grave and mortal sin is. Definitions may only lead to more discussion about words and terms.
But in order to know what a grave and mortal sin is, we just have to take a look at the 1st reading of today.
Acts like fornication, gross indecency and sexual irresponsibility, idolatry and sorcery, feuds and wrangling, jealousy, bad temper and quarrels, etc., and similar things do not need much explanation and definition.
When self-indulgence is at work these are what will happen and it is obvious what kind of morality they belong to.
As much as the morality of these acts are grave and mortal, yet in the gospel Jesus pointed out something else that is not so obvious.
Self-indulgence will lead to grave and mortal sin. Yet self-indulgence will also lead to other sins that are not so obvious like hypocrisy and oppression and injustice and deception.
Jesus is not saying that these are lesser sins. In fact He had harsh words for the Pharisees and the lawyers of the Law for committing such sins and getting away with it.
As the 1st reading puts it, if we are led by the Spirit, then no law can touch us because the Spirit will be our life and we will be directed by the Spirit.
We have to crucify all self -indulgent passions and desires if we want to be lead by the Spirit and truly belong to Christ.
Then we will be able to bear the fruits of the Spirit - love, joy, peace, patience, kindness, goodness, trustfulness, gentleness and self-control.