(you)whose power derives from the fullness of grace that Christ has entrusted to the Catholic Church So am I to understand that salvation can come either from the Roman Catholic church or from other Christian communities? And that the salvation from other Christian communities is not full salvation but something less because it’s not Roman Catholic?
I wonder if this is really a serious question, since this issue has been addressed many times by a number of very capable Catholics on the FR religion forum. But, on the chance that this is on the level, I will take a stab at it now.
There is a difference between "fullness of the means of salvation" and whether or not a non-Catholic Christian can be saved at all. Simply parsing out the English words here should give the sense of that.
Most of what I am going to say follows logically from the Catholic Church's self-awareness as the Church that was personally founded by Jesus Christ. I am aware that, by definition, I suppose, you would contend such is not the case. Nevertheless, if we are to get anywhere on this, you have to understand the Church's self-perception here.
So, if the Catholic Church was, in fact, founded by Jesus, and if He endowed it with the Sacraments that are the primary means of bestowing Sanctifying Grace and Sacramental Grace to our souls, which, from a Catholic perspective, are necessary to enter into Heaven, then it follows that any church that does not maintain these Sacraments, at the very least, does not make available the "fullness of the means of salvation." Only the Catholic and Eastern Orthodox Churches maintain all of the Sacraments, and with a view that they supply grace, therefore, by definition, all other non-Catholic bodies, from our POV, do not maintain all of the Sacraments and/or do not maintain a catholic understanding of what they are and what they do. I don't ask you to agree with the Church's conclusion here (though you should!), I am only asking you to see this, for sake of argument anyway, from the Catholic POV in order to understand what is being said.
Nowhere in all of this is there an argument about, as you put it, the "fullness of salvation." One is either saved or is not saved, there are no gradations of salvation. Therefore, you are not understanding the question properly. Catholics acknowledge Trintiarian baptism as valid, even in Protestant Churches. Therefore, remission of sins has taken place, and sanctifying grace is supplied to the recipients (even if they deny the concept) at the time of Baptism. Therefore, any Christian, at the moment of baptism, has sanctifying grace in his or her soul, and, were such a one to die at that moment, would surely be saved.
However, most people live long after baptism! They commit subsequent sins. Those sins that St. John speaks about in 1John 5:16-17 as "mortal" or "deadly" will preclude one's entry into Heaven without subsequent forgiveness. That is why they are called "mortal sins." They spell spiritual death. From the Catholic perspective, those sins need to be sacramentally confessed in order to restore us to a state of justification and for sanctifying grace to be granted to us again. Here is where there is a real problem for non-Catholic Christians, from a Catholic POV. If you don't believe in sacramental confession, and, therefore, you don't avail yourself of it, the "ordinary" means of securing God's forgiveness are not available to you. Therefore, it will be exceedingly difficult for you to become "justified" in the sight of God again. Note, however, that I did not say "impossible." Even Catholics, who, in their dying hour, repent of their sins with what is called " perfect contrition," but don't have a priest available, can make a "perfect act of contrition" and become justified again. This is not easy, however. For this, one must be not only sorry for their sins because of a fear of damnation, one must be sorry for his or her sins iwht perfect contrition for having offended God. This means also having contrition for even the slightest attachment to their sins, in addition to the sins themselves. Again, this is not easy!
It is on this basis that the Church does not say that it is "impossible" for non-Catholic Christians to be saved, but it is quite difficult, when A) most non-Catholics aren't even consciously aware of this, and B0 in anycase, it is far better, from a Catholic POV, to die with the graces of the Sacraments. That is why the "fullness of the means of salvation" is so important.
Again, you doubtless don't agree with any of this. But that's not what is at issue, or, at least, based on you questions, it should not be at issue. I am trying to give you an answer from the source POC - the Catholic Church. You may not agree with the premises, but I hope you can see that "the fullness of the means of salvation" is not the same as "the fullness of salvation," and that, in any event, "fullness of salvation" makes no sense, since you either have salvation when judged by God at death or you don't.
Again, thanks.