Grave sin, Mortal sin, and Serious sin
Are different words for the same thing.
The Compendium of the Catechism
"304. Which sins must be confessed?
1456
All grave sins not yet confessed, which a careful examination of conscience brings to mind, must be brought to the sacrament of Penance. The confession of serious sins is the only ordinary way to obtain forgiveness."
(note the use of the term grave sins and serious sins …in the same breath.)
ECCLESIA DE EUCHARISTIA --Pope John Paul II
“If a Christian’s conscience is burdened by serious sin, then the path of penance through the sacrament of Reconciliation becomes necessary for full participation in the Eucharistic Sacrifice.”
Pope John Paul II Reconciliatio et Paenitentia 17.
“Considering sin from the point of view of its matter, the ideas of death, of radical rupture with God, the supreme good, of deviation from the path that leads to God or interruption of the journey toward him (which are all ways of defining mortal sin) are linked with the idea of the gravity of sin’s objective content. Hence, in the church’s doctrine and pastoral action, grave sin is in practice identified with mortal sin.”
In the Message of Pope John Paul II to the participants of a course on confession organized by the Apostolic Penitentiary in March 1998
“The sacrament of Penance is meant to take away personal sins committed after Baptism: first of all mortal sins, then venial. If the penitent has committed more than one mortal sin, they can only be remitted all at once. In fact, the remission of serious sin consists in the infusion of the sanctifying grace which has been lost, and grace is incompatible with any and every serious sin. Venial sins are to be regarded differently…”
Notice he used “mortal sin and serious sin” interchangably.
Pope Benedict XVI in his Catecheses with Children said:
"Only in that case, when you are in a state of “mortal” sin, in other words, grave (sin), is it necessary to go to confession before Communion. This is my first point. "
vatican.va/holy_father/benedict_xvi/speeches/2005/october/documents/hf_ben_xvi_spe_20051015_meeting-children_en.html
and here
“The faithful, in their turn, must seek to receive and to venerate the Most Holy Sacrament with piety and devotion, eager to welcome the Lord Jesus with faith, and having recourse, whenever necessary, to the sacrament of reconciliation so as to purify the soul from every grave sin.”
vatican.va/holy_father/benedict_xvi/homilies/2007/documents/hf_ben-xvi_hom_20070511_canonization-brazil_en.html
There Pope Benedict refers to "grave sin’.
In regards to what an action is:
Murder, adulty, robbing a bank, etc are all “Mortal sins, grave sins, or serious sins” one can use either term.
And regarding as to of a person actually commits a Mortal sin.
this can also be referred to as a grave sin or a serious Sin.
This is how the Church uses the terms and that is how the get translated in official documents into English.
So when Canon law refers to being ‘conscious of grave sin’ it means that the person is conscious of actually committing a grave sin with full knowledge and full consent.
One can call it a grave sin or a mortal sin or a serious sin. same thing that the person is conscious of.
Yes if done knowingly it is a sacrilege. (I used the word can deliberately – for there are cases when the Church says if one has a grave reason to go and one can not get to confession one is to make a pefect act of contrition and then they may go --but this is NOT the norm…it is for example when a person lives where a priest can only come every 6 months…and they have a grave reason …that sort of thing THEN they do not commit sacrilege.