Yes, true enough - this is because of their character, since their commission is visited with "the undying penalty of hell". But to be more correct - we say in the absence of the sacrament or at least a true desire for its reception, coupled with perfect contrition and love of God.
At any rate, St. John makes the same distinction as we do: sins which are punished temporally with limited punishments, and sins which are punished eternally with hell.
Mark and Bessarion's argument is interesting (as I thought when I read the piece some time ago). But their linguistic arguments don't quite hold up. Liddell and Scott give, for sozo (the word used in 1 Cor. 3:15) two separate sets of definitions. The first, "of persons, to save from death, keep alive, preserve", the second,
II. with a sense of motion to a place, to bring one safe to, ton d' esaôsen es potamou prochoas Od.; sô. tina pros êpeiron Aesch.:--in Pass. to come safe, escape to a place, es oikon Hdt.; epi thalattan Xen.
2. to carry off safe, rescue from danger, ek polemou Il.; ek thanatoio Od.; apo strateias Aesch.:--c. gen., echthrôn sôsai chthona to rescue the land from enemies, Soph.; Pass., sôthênai kakôn Eur.
3. c. inf., hai se sôzousin thanein who save thee from dying, id=Eur.
4. absol., ta sôsonta what is likely to save, Dem.
So it can mean either. The same goes for zemiothesetai, which can also mean "to be punished" or "to be fined", which accord quite well with our doctrine.
St. Thomas argues that the "wood, hay, stubble" must be venial sins, since it would be impossible to build mortal sins on the foundation of Christ (in his commentary on the text):
Therefore, some, referring this to the superstructure of works, have said the gold, silver and precious stones mean the good works a person adds to his faith; but wood, hay and stubble mean the mortal sins a person commits after receiving the faith.However, this interpretation cannot stand: first, because mortal sins are dead works: “He will cleanse our consciences from dead works” (Heb 9:14), whereas only living works are built onto this building: “Be you also as living stone built up” (1 Pt 2:5). Consequently, those who have mortal sins along with faith do not build up, but rather destroy or profane. Against such persons he says: “But if anyone destroys God’s temple” (1 Cor 3:17).
Secondly, because mortal sins are better compared to iron or lead or stone, since they are heavy and not destroyed by fire but always remain in the thing in which they exist; whereas venial sins are compared to wood, hay and stubble, because they are light and easily cleansed from a person by fire.
Thirdly, because it seems to follow from this interpretation that a person who dies in mortal sin, as long as he keeps the faith, will finally attain to salvation after undergoing punishment. For he continues: If any man’s work is burned up he will suffer loss, though he himself will be saved, but only as through fire, which is obviously contrary to the Apostle’s statement below (6:9): “neither fornicators, nor idolaters, nor adulterers, nor homosexuals....shall posses the kingdom of God”, and to Gal (5:21): “Those who do such things shall not possess the kingdom of God.” But one possesses salvation only in the kingdom of God; for everyone excluded from it is sent into eternal fire, as it says in Matt (25:41).
Fourthly, because faith can be called a foundation, only because by it Christ dwells in us, since it was stated that the foundation is Christ Jesus Himself. For Christ does not dwell in us by unformed faith; otherwise He would dwell in the devils, of whom Jas (2:19) says: “The devils believe and tremble.” Hence Eph (3:17) says: “that Christ by faith may dwell in your hearts.” This should be understood of faith informed by charity, since 1 Jn (4:16) says: “He that abides in love abides in God and God in him.” This is the faith that works through love, as it says below (13:4): “Love is not arrogant or rude.” Consequently, it is obvious that persons who commit mortal sins do not have formed faith, and so do not have the foundation. Therefore, it is necessary to suppose that the person who builds upon the foundation gold, silver and precious stones, as well as one who builds upon it wood, hay, stubble, avoids mortal sin.
"Mark and Bessarion's argument is interesting (as I thought when I read the piece some time ago). But their linguistic arguments don't quite hold up."
Here's the problem. +Mark and Bessarion spoke Greek and they believed they were "refuting" certain arguments of the Latins based in a selective reading of some Fathers along with some mistranslations. Now of course, maybe they didn't understand their own language, but I doubt it. I say this particularly because Bessarion was in favor of the reunion and was seeking a common ground between the East and the West but soundly condemned and continued to condemn the Latins for their sorry lack of knowledge and understanding of Greek.
Its funny you should mention this linguistics matter. I just got back from Greece where I was in a discussion of some Church matters involving a local situation of no great theological import but of practical importance to the people involved. At any rate, there was an English fellow there who appropriately took part in the discussion. His Greek was pretty good, but it was the Greek a foreigner learns in school and then applies on site over a period of time. Some of what he said, while gramtically correct, taken in the context of the time place and culture made no sense. In fact, one of the Greeks commented to me that the man was an idiot. He isn't an idiot, he just wasn't thinking like a Greek speaker when he spoke Greek. I think he thought he was quite profound...but he wasn't.
Its like a number of us have said before, sometimes we use different words to describe the same thing and at others the same words to describe very different things. I suggest your reliance on Liddell and Scott may be misplaced.