JoeZ,
Thank you for the quote. There is no doubt that among those who die during instruction before receiving baptism there would be - indeed, I would say the vast majority - those who do not have the faith and heigh level of contrition (probably would need to be "perfect" contrition) to be justified by their vow or desire for the sacrament. To those indeed the sacrament would be necessary, as the sacrament of penance is necessary for those who have, say imperfect contrition for sins committed - the imperfect contrition is enough for the justification of the sacrament of penance, but not for the justification before receipt of the sacrament that comes by way of perfect contrition. Think about it: obviously someone who is justified in the sacrament of penance via an imperfect contrition didn't have perfect contrition before the sacrament.
I would say it is the same regarding baptism. There are those who come to the fount like those who come to the sacrament of penance with imperfect contrition: they would not justified without the sacrament. For them, the receipt of the sacrament is a necessity.
Pope Siricius speaks of infants, and the aforementioned are like infants in that the receipt of the sacrament is absolutely necessary for them, just as it is for infants. They are therefore spoken of collectively by the pope, and for both alike the sacraments are necessary. Remember the language in the Catechism of Trent:
This is consistent with what I am saying. If death is imminent for an adult, mercy says give the sacrament - they may be those who, as I have argued, lack the level of contrition or faith that would "avail them to grace and righteousness," and they would be in the same boat as the infant.
I see an argument against my position by saying that the Catechism implies that a BoD would apply to all catechumen in such circuмstances. But I don't think it can be read that way, and think my analogy to the sacrament of penance applies. As quoted in the op, the Council says that "This sacrament of penance, moreover, is necessary for the salvation of those who have fallen after baptism, as baptism itself is for those as yet not regenerated." The same necessity for justification: the actual receipt of the sacrament is necessary for those with imperfect contrition, and I would say the same for some catechumen who do have an imperfect contrition for their past sins. There are some catechumen, I would say, whose "intention and determination" is not perfect and requires the sacrament of baptism.
In sum, I do not think the quotes from Pope Siricius address general sacramental necessity, like Trent does, but a sacramental necessity for those in a certain situation, a dire circuмstance. You don't play "Russian Roulette" and hope those people have "perfect contrition," like you don't delay with infants.
A good quote and very helpful for thinking about this, but I don't see it as having the generality of Trent, or addressing generally the necessity of the sacraments. Pope Siricius is addressing a specific circuмstance in which baptism should not be delayed for adults, which is acknowledged and addressed also in the Catechism of Trent, which talks about BoD or justification by votum with contrition as well.
DR
[Emphasis added above.]
A cautionary side comment:
Be very careful in the use of analogy. This works only as an illustrative didactic tool where there is a clear correspondence, not where certain attributes of the items being compared might elicit an inapplicable premise.
Penance is Penance. Baptism is Baptism. Penance requires reason and the capacity to accuse oneself in detail after purposeful reflection. This requirement does not apply to Baptism per se, as is obvious in the section from Pope Siricius (emphasis added):
Baptism Of Adults
With regard to those of adult age who enjoy the perfect use of reason, persons, namely, born of infidel parents, the practice of the primitive Church points out that a different manner of proceeding should be followed. To them the Christian faith is to be proposed; and they are earnestly to be exhorted, persuaded and invited to embrace it.
So while I'm not prepared to comment on necessity, I will point again to the problem of desire/intent/
votum. It's one thing to discuss the generalities of sacraments, but it's another entirely to impute particular
votum and then ride upon that.
Votum for Penance is its own thing, and it doesn't serve to explicate desire for Baptism.