Would it be mortally sinful for a priest to add a prayer for example into the liturgy or participate in some of the common liturgical abuses (breaking host at consecration, priest holding hands at Lord’s Payer, etc)?
Objectively yes, subjectively due to mitigating circumstances God is the judge.
The same conditions for mortal sin apply for the priest at Mass as for any other sin.
Was it gravely wrong? In virtually all cases, the answer would be yes since beyond the instructions for the Mass, the priest would be guilty of the sin of disobedience - grave matter for a priest.
Did he know it was wrong? I can’t imagine a scenario where the priest knows the Mass and reads from the Missal but has never read the GIRM.
Did he act with full consent? Here’s where we just don’t know. There are priests “of a certain age” that were poorly formed and who developed some very bad habits and may have a malformed conscience regarding Liturgy. Others just don’t think the rules apply to them.
I have heard many priests and Bishops say that they have been taught that judgement will be tougher for the ordained because they have been given much grace and with that comes more responsibility.
Pray for all of our priests.
But in that case, it would be accidental and you can’t sin accidentally.
Correct, which means that it wouldn’t be a sin.
You can’t mortally sin accidentally.
Depends what exactly. There are certain Rites that can be integrated into Liturgy and would substitute for parts of the Liturgy, that is not an abuse. Just an example that the Liturgy isn’t a straight-foward, only one way to do it.
There are also parts of the Liturgy that has some freedom. The Homily for example, allows for the priest to do a wide variety of things to make his teaching. In the Prayers of the Faithful, you can add petitions.
In cases of abuse, it can be minor abuse to grave abuse. Some needs to be explicitly stated while others would be obvious. In the case of the breaking of the Host at the Consecration, I think the Church has already stated its a major abuse.
So a major or grave abuse would be a mortal sin (diobedience to Church) while some abuses are minor.
I have a bit more time now. Consider
You are carrying a loaded gun. Safety off. Being very careful. BUT earthquake, you drop the gun, it goes off and kills someone.
A sin, not mortal but, because you could have foreseen the consequences, you could have prevented it. You chose NOT to prevent. No intention to commit. So you accidentally commited a venial sin.
Or suppose my wife says ‘Does this make my butt look fat?’ and, without thinking, I say, ‘sort of’. She is deeply hurt. I sinned against charity, in that what I said was hurtful (and truthful). No intention, accidentally. But still a venial sin.
The one thing I don’t really get is why would a priest want to change the words of a mass at all. Isn’t it enough easy just to read the words off from the liturgal booklet? Honestly, I cannot think of a better reason other than misguiding the faithful.
As for the additional prayers, I think as long as the bishop of the diocese agree upon then it’s fine.
Changing the words I agree with, the only reason is either misguided or nefarious. However, priests (and sometimes the faithful) are allowed to add things spontaneously (for example, at the end of the Prayers of the Faithful, a prayer like a Hail Mary may be added as long as it is spontaneous).
In your first example, it’s not the venial sin of killing someone. It’s a different sin of negligence. The result (killing) is not the sin.
In your second example, you may have sinned against prudence but telling the truth is not sinful.
This is probably the wrong post for this discussion. I disagree with you, but this is the wrong place to discuss it so I will let it go.
Due to Pope Benedict’s call for Mutual Enrichment, although in reality it only goes one way, a Priest could add some prayers to the Mass. The Pope has done it. Specifically the incensation of the altar prayers. Another example would be the Oramus te when kissing the altar.
Please see Fr. Finigan’s article.
A priest cannot change any prayers written in the Missal in order to adjust it to his homily for example.